the circadian rhythm reset

The science behind the Reset

You don’t just need to take my word for what I'm teaching. There is an abundance of science to back up everything I teach in The Circadian Rhythm Reset, so I compiled it all together so you can see all of it in one place.

A lot of the circadian advice out there is ‘do this, do this, do this,’ with nothing substantial underneath it to back it up. Everything in the Reset sits on real research. Some of it I walk you through right inside the lessons, like the camping study that moved people’s clocks two hours in one week, or the 71,000 nurses whose menstrual cycles dramatically changed with the chronic circadian disruption of night shifts. The rest sits behind the numbers and statistics that you'll hear me say.

So here are all 64 sources, in the order the course teaches them, each with a line on what it found and a link so you can read the study yourself.

Nerd out as much as you want. And if you’d rather see how it all comes together in your own days, that’s exactly what the Reset is for. (So you don't actually have to read all these papers too lol).

Back to The Circadian Rhythm Reset

Module 1: The Foundation

Lesson 1.1: Why Everything Feels Off at Once

  1. A woman's body is made of about 28 trillion cells.Hatton IA, et al. The human cell count and size distribution. PNAS, 2023. doi.org/10.1073/pnas.2303077120
  2. Even red blood cells, which have no nucleus, keep their own daily rhythm, so nearly every one of those cells keeps its own time.O'Neill JS, Reddy AB. Circadian clocks in human red blood cells. Nature, 2011. doi.org/10.1038/nature09702

Lesson 1.2: It's Not You, It's the Light

  1. Your eyes have a third kind of light receptor, made with a protein called melanopsin, and it's the main way light sets your body clock. It's most sensitive to blue light around 480 nanometers.Lucas RJ, et al. Measuring and using light in the melanopsin age. Trends in Neurosciences, 2014. doi.org/10.1016/j.tins.2013.10.004
  2. Light landing on the lower half of the retina, which is the light coming from above, lowered melatonin. The same light on the upper half barely did.Glickman G, et al. Inferior retinal light exposure is more effective than superior retinal exposure in suppressing melatonin in humans. Journal of Biological Rhythms, 2003. doi.org/10.1177/0748730402239678
  3. At sunrise, a clear sky already gives about 400 to 600 lux.American Meteorological Society. Civil twilight. Glossary of Meteorology. glossary.ametsoc.org/wiki/Civil_twilight
  4. Within about an hour of sunrise, it's around 10,000 lux. Clouds turn it down, to about half under thin clouds and as little as a tenth under heavy, dark skies.Janiczek PM, DeYoung JA. Computer programs for sun and moon illuminance with contingent tables and diagrams. U.S. Naval Observatory Circular No. 171, 1987. archive.org/details/DTIC_ADA182110
  5. Left on its own, the human body clock runs a little longer than 24 hours.Czeisler CA, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 1999. doi.org/10.1126/science.284.5423.2177
  6. Women's clocks run a little longer than 24 hours on average, too.Duffy JF, et al. Sex difference in the near-24-hour intrinsic period of the human circadian timing system. PNAS, 2011. doi.org/10.1073/pnas.1010666108
  7. Even the brightest full moon in decades gave only about 0.3 lux.Kyba CCM, Mohar A, Posch T. How bright is moonlight? Astronomy & Geophysics, 2017. doi.org/10.1093/astrogeo/atx025
  8. A candle gives about 1 lux at a meter away, and Edison's early bulbs about 16. A history of how much light people had, from fire to electricity.Nordhaus WD. Do real-output and real-wage measures capture reality? The history of lighting suggests not. In The Economics of New Goods, University of Chicago Press, 1996. nber.org/chapters/c6064
  9. LED bulbs first reached stores around 2007 to 2009, and through the 2010s they took over American homes.U.S. Department of Energy. Adoption of light-emitting diodes in common lighting applications. 2017. energy.gov/sites/prod/files/2017/08/f35/led-adoption-jul2017_0.pdf
  10. A white LED is a blue chip under a yellow coating, which is why its light carries a sharp spike of blue near 450 nanometers.Kusuma P, Pattison PM, Bugbee B. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research, 2020. doi.org/10.1038/s41438-020-0283-7
  11. Blue light, around 446 to 477 nanometers, lowered melatonin at night more than any other color, in 72 adults tested across eight colors of light.Brainard GC, et al. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. Journal of Neuroscience, 2001. doi.org/10.1523/JNEUROSCI.21-16-06405.2001
  12. A second lab found the same blue peak that year, and showed it wasn't the rods or cones we see with.Thapan K, Arendt J, Skene DJ. An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. Journal of Physiology, 2001. doi.org/10.1111/j.1469-7793.2001.t01-1-00261.x
  13. A phone at reading distance puts about 40 to 50 lux into your eyes, and every LED screen tested had the same blue spike.Gringras P, et al. Bigger, brighter, bluer-better? Current light-emitting devices: adverse sleep properties and preventative strategies. Frontiers in Public Health, 2015. doi.org/10.3389/fpubh.2015.00233
  14. Our night skies are brightening so fast that a child born today could lose more than half the stars she can see by the time she turns 18 (the lead researcher's projection from citizen scientists' measurements).Kyba CCM, et al. Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022. Science, 2023. doi.org/10.1126/science.abq7781
  15. Indoors by day, the light reaching our eyes is usually about 20 to 100 lux at home, dimmer than the sky at sunrise.Diver WR, et al. Evaluation of a novel ambient light survey question in the Cancer Prevention Study-3. Int J Environ Res Public Health, 2023. doi.org/10.3390/ijerph20043658
  16. The camping study. After one week of camping with only sunlight and firelight, people's body clocks shifted about two hours earlier.Wright KP Jr, et al. Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology, 2013. doi.org/10.1016/j.cub.2013.06.039
  17. Night owls are partly real: the longer someone's inner clock runs, the later they tend to wake and the more of an evening person they are.Duffy JF, Rimmer DW, Czeisler CA. Association of intrinsic circadian period with morningness-eveningness, usual wake time, and circadian phase. Behavioral Neuroscience, 2001. doi.org/10.1037/0735-7044.115.4.895

Lesson 1.3: Wired, Tired, Inflamed, Stressed & Hormonal

  1. Ordinary room light before bed (under 200 lux) delayed melatonin's rise in nearly everyone and shortened the body's melatonin night by about 90 minutes. The same light left on during sleep hours cut melatonin by more than half in most people.Gooley JJ, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab, 2011. doi.org/10.1210/jc.2010-2098
  2. Reading on a light-emitting tablet for a few hours before bed suppressed evening melatonin by about 55% and pushed the body clock more than an hour and a half later.Chang AM, et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS, 2015. doi.org/10.1073/pnas.1418490112
  3. One hour of light at about 800 lux after waking raised morning cortisol by about 35%.Scheer FAJL, Buijs RM. Light affects morning salivary cortisol in humans. J Clin Endocrinol Metab, 1999. doi.org/10.1210/jcem.84.9.6102
  4. The brain makes more serotonin on brighter, sunnier days.Lambert GW, et al. Effect of sunlight and season on serotonin turnover in the brain. The Lancet, 2002. doi.org/10.1016/S0140-6736(02)11737-5
  5. Melatonin is more than a sleep hormone. It gathers inside the mitochondria of your cells and protects them from damage, and the body makes most of it at night.Reiter RJ, et al. Melatonin as a mitochondria-targeted antioxidant: one of evolution's best ideas. Cellular and Molecular Life Sciences, 2017. doi.org/10.1007/s00018-017-2609-7
  6. Melatonin turns down one of the body's main inflammation switches, across 38 lab and animal studies.Arioz BI, et al. The role of melatonin on NLRP3 inflammasome activation in diseases. Antioxidants, 2021. doi.org/10.3390/antiox10071020
  7. In 31 human trials, melatonin lowered several markers of inflammation.Cho JH, et al. Anti-inflammatory effects of melatonin: a systematic review and meta-analysis of clinical trials. Brain, Behavior, and Immunity, 2021. doi.org/10.1016/j.bbi.2021.01.034
  8. Just three days living out of step with their body clocks raised shift workers' inflammation marker, CRP, by 11%.Morris CJ, et al. Circadian misalignment increases C-reactive protein and blood pressure in chronic shift workers. Journal of Biological Rhythms, 2017. doi.org/10.1177/0748730417697537
  9. When mice lived through weeks of repeated jet lag, the clocks in their organs and immune cells fell out of rhythm, and their immune systems overreacted.Castanon-Cervantes O, et al. Dysregulation of inflammatory responses by chronic circadian disruption. Journal of Immunology, 2010. doi.org/10.4049/jimmunol.1001026
  10. Cycles and the moon. In the long-term period records of 176 women, cycles were in sync with the full and new moon before 2010. After LEDs and smartphones arrived, that sync all but disappeared.Helfrich-Förster C, et al. Synchronization of women's menstruation with the Moon has decreased but remains detectable when gravitational pull is strong. Science Advances, 2025. doi.org/10.1126/sciadv.adw4096
  11. The Nurses' Health Study II. Among more than 71,000 women, nurses who had worked rotating night shifts for at least 20 months were 23% more likely to report irregular cycles and 49% more likely to report cycles of 40 days or longer, and the more months of night shifts, the bigger the change.Lawson CC, et al. Rotating shift work and menstrual cycle characteristics. Epidemiology, 2011. doi.org/10.1097/EDE.0b013e3182130016
  12. The surge that leads to ovulation most often begins in the early morning hours, between about midnight and 8 a.m.Cahill DJ, et al. Onset of the preovulatory luteinizing hormone surge: diurnal timing and critical follicular prerequisites. Fertility and Sterility, 1998. doi.org/10.1016/S0015-0282(98)00113-7
  13. When researchers removed the master clock in rats, they stopped ovulating, even with the rest of their reproductive hormones still there.Wiegand SJ, Terasawa E. Discrete lesions reveal functional heterogeneity of suprachiasmatic structures in regulation of gonadotropin secretion in the female rat. Neuroendocrinology, 1982. doi.org/10.1159/000123335
  14. The ovaries concentrate melatonin in the fluid around a developing egg, where it protects the egg through ovulation.Tamura H, et al. The role of melatonin as an antioxidant in the follicle. Journal of Ovarian Research, 2012. doi.org/10.1186/1757-2215-5-5
  15. After ovulation, melatonin protects the corpus luteum and helps it keep making progesterone.Taketani T, et al. Protective role of melatonin in progesterone production by human luteal cells. Journal of Pineal Research, 2011. doi.org/10.1111/j.1600-079X.2011.00878.x
  16. PCOS has a new name: PMOS, polyendocrine metabolic ovarian syndrome.Teede HJ, et al. Polyendocrine metabolic ovarian syndrome, the new name for polycystic ovary syndrome: a multistep global consensus process. The Lancet, 2026. doi.org/10.1016/S0140-6736(26)00717-8
  17. After menopause, women's daily rhythms were flatter and earlier, and their sleep more disturbed, in a study of 177 women.Gómez-Santos C, et al. Menopause status is associated with circadian- and sleep-related alterations. Menopause, 2016. doi.org/10.1097/GME.0000000000000612
  18. Shifts in the body clock and lower melatonin are part of why sleep gets harder in perimenopause.Troìa L, et al. Sleep disturbance and perimenopause: a narrative review. Journal of Clinical Medicine, 2025. doi.org/10.3390/jcm14051479
  19. In women in and after perimenopause, morning light that moved their melatonin rhythm earlier lifted depression (a small, early study).Parry BL, et al. Sleep-light interventions that shift melatonin rhythms earlier improve perimenopausal and postmenopausal depression: preliminary findings. Menopause, 2023. doi.org/10.1097/GME.0000000000002216

Module 2: A Circadian Day

Lesson 2.1: Follow the Sun

  1. College students on irregular sleep schedules started making melatonin about two and a half hours later than students on regular ones.Phillips AJK, et al. Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Scientific Reports, 2017. doi.org/10.1038/s41598-017-03171-4

Lesson 2.2: Your Sun

  1. Red and near-infrared light act on the energy machinery inside your mitochondria, the main idea behind red light therapy.Hamblin MR. Mechanisms and mitochondrial redox signaling in photobiomodulation. Photochemistry and Photobiology, 2018. doi.org/10.1111/php.12864
  2. The near-infrared in sunlight passes right through the human body, and in a small study, that kind of light improved vision a day later.Jeffery G, et al. Longer wavelengths in sunlight pass through the human body and have a systemic impact which improves vision. Scientific Reports, 2025. doi.org/10.1038/s41598-025-09785-3

Lesson 2.3: Your Space

  1. Two white lights with the same brightness and the same Kelvin number had very different effects on melatonin.Souman JL, et al. Spectral tuning of white light allows for strong reduction in melatonin suppression without changing illumination level or color temperature. Journal of Biological Rhythms, 2018. doi.org/10.1177/0748730418784041
  2. When offices swapped flickering fluorescent lights for flicker-free ones, headaches and eyestrain fell by more than half, though no one could see the flicker.Wilkins AJ, et al. Fluorescent lighting, headaches and eyestrain. Lighting Research & Technology, 1989. doi.org/10.1177/096032718902100102
  3. Your retina keeps responding to flicker far faster than you can see, up to around 160 times a second.Berman SM, et al. Human electroretinogram responses to video displays, fluorescent lighting, and other high frequency sources. Optometry and Vision Science, 1991. doi.org/10.1097/00006324-199108000-00012
  4. Nerve cells in the eye and brain fire in time with flicker that's too fast to see (a study in cats).Eysel UT, Burandt U. Fluorescent tube light evokes flicker responses in visual neurons. Vision Research, 1984. doi.org/10.1016/0042-6989(84)90069-5
  5. Some LEDs flicker a lot and some barely at all.Miller NJ, et al. Flicker: a review of temporal light modulation stimulus, responses, and measures. Lighting Research & Technology, 2023. doi.org/10.1177/14771535211069482
  6. Even at a dim evening brightness, cooler, bluer light held back melatonin's rise more than warmer light did.Chellappa SL, et al. Non-visual effects of light on melatonin, alertness and cognitive performance: Can blue-enriched light keep us alert? PLoS ONE, 2011. doi.org/10.1371/journal.pone.0016429

Lesson 2.4: Your Screens

  1. Red light at night left melatonin alone, but bright enough red still made people a little more alert (one of a few small studies).Figueiro MG, et al. Preliminary evidence that both blue and red light can induce alertness at night. BMC Neuroscience, 2009. doi.org/10.1186/1471-2202-10-105
  2. An hour of red light at night lowered sleepiness a little, the second of those small studies.Plitnick B, et al. The effects of red and blue light on alertness and mood at night. Lighting Research & Technology, 2010. doi.org/10.1177/1477153509360887

Also in Module 2, from Module 1: the morning cortisol study (22) in 2.2a; room light before bed and during sleep (20) in 2.3a and 2.3d; cycles and the moon (29) in 2.3d; light from above and the lower retina (4) in 2.5b.

Module 3: Living in Rhythm

Lesson 3.2: A Rhythm You Can Live

  1. Fatal car crashes in the U.S. rise about 6% in the week after we spring forward, across 22 years of data.Fritz J, et al. A chronobiological evaluation of the acute effects of daylight saving time on traffic accident risk. Current Biology, 2020. doi.org/10.1016/j.cub.2019.12.045
  2. Heart attacks jumped about 24% on the Monday after springing forward, across more than 42,000 hospital admissions in Michigan.Sandhu A, Seth M, Gurm HS. Daylight savings time and myocardial infarction. Open Heart, 2014. doi.org/10.1136/openhrt-2013-000019
  3. The American Academy of Sleep Medicine has called for ending the clock changes, in favor of permanent standard time.Rishi MA, et al. Daylight saving time: an American Academy of Sleep Medicine position statement. Journal of Clinical Sleep Medicine, 2020. doi.org/10.5664/jcsm.8780

Lesson 3.3: Follow the Sun Through the Year

  1. In people living on winter-length nights for several weeks, the melatonin night grew by about an hour and a half to two hours.Wehr TA. The durations of human melatonin secretion and sleep respond to changes in daylength (photoperiod). J Clin Endocrinol Metab, 1991. doi.org/10.1210/jcem-73-6-1276
  2. And the night-time stretch of sleep, body temperature and cortisol grew right along with it.Wehr TA, et al. Conservation of photoperiod-responsive mechanisms in humans. American Journal of Physiology, 1993. doi.org/10.1152/ajpregu.1993.265.4.R846
  3. The signal so many animals use as their calendar, the length of the melatonin night, still appears to be there in us, from the eye all the way to the melatonin.Wehr TA. Photoperiodism in humans and other primates: evidence and implications. Journal of Biological Rhythms, 2001. doi.org/10.1177/074873001129002060
  4. About a quarter of the genes tested in children's immune cells changed their activity with the seasons, and in Australia, where the seasons are flipped, the pattern flipped too.Dopico XC, et al. Widespread seasonal gene expression reveals annual differences in human immunity and physiology. Nature Communications, 2015. doi.org/10.1038/ncomms8000
  5. Across 329,261 adults, immune cells and a key marker of inflammation rose and fell with the time of year.Wyse C, et al. Seasonal and daytime variation in multiple immune parameters in humans: evidence from 329,261 participants of the UK Biobank cohort. iScience, 2021. doi.org/10.1016/j.isci.2021.102255
  6. In the darker months, the brain holds on to more of the transporters that clear serotonin away.Praschak-Rieder N, et al. Seasonal variation in human brain serotonin transporter binding. Archives of General Psychiatry, 2008. doi.org/10.1001/archpsyc.65.9.1072
  7. The winter campers. When the same researchers took people camping around the winter solstice, their melatonin night stretched to about fourteen hours, compared with about ten for the summer campers.Stothard ER, et al. Circadian entrainment to the natural light-dark cycle across seasons and the weekend. Current Biology, 2017. doi.org/10.1016/j.cub.2016.12.041
  8. Your skin can only make vitamin D once the sun climbs high enough in the sky. This study mapped when that happens, by latitude and season.Engelsen O, et al. Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness. Photochemistry and Photobiology, 2005. doi.org/10.1562/2004-11-19-RN-375
  9. Winter sun in Boston made no vitamin D in skin from November through February, and in Edmonton, from October through March.Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab, 1988. doi.org/10.1210/jcem-67-2-373
  10. As the sun sinks lower, the UV that makes vitamin D fades faster than the UV that burns.McKenzie RL, Liley JB, Björn LO. UV radiation: balancing risks and benefits. Photochemistry and Photobiology, 2009. portal.research.lu.se/en/publications/uv-radiation-balancing-risks-and-benefits/

Lesson 3.5: The Inward Seasons: Autumn and Winter

  1. After dim winter days, the same evening light suppressed melatonin almost twice as much in winter as in summer.Higuchi S, et al. Less exposure to daily ambient light in winter increases sensitivity of melatonin to light suppression. Chronobiology International, 2007. doi.org/10.1080/07420520601139805
  2. In two communities that still live without electricity, people slept about an hour more in winter than in summer.Yetish G, et al. Natural sleep and its seasonal variations in three pre-industrial societies. Current Biology, 2015. doi.org/10.1016/j.cub.2015.09.046

Also in Module 3, from Module 1: the camping study (18) in 3.3a; serotonin and sunshine (23) in 3.3a; the long clock (7, 8) in 3.5a, falling back.

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