Circadian Rhythm and Longevity: How Light, Meal Timing, and Social Jet Lag Age You
Most cells in the body contain their own 24-hour clock, coordinated by a master clock in the brain and kept in step with the day mainly by light.1,3 When eating, sleep and light exposure fall out of step with these clocks, metabolism and other processes can suffer.
- The clock is not just a sleep-wake switch. The master clock drives daily rhythms in activity and rest, feeding, body temperature and hormones, and the clocks in other tissues are closely tied to metabolism.1
- Social jet lag is the gap between the sleep timing your body clock prefers and the timing your schedule demands. In a large epidemiological study, more social jet lag was associated with a higher body mass index, beyond the effect of sleep duration.5
- Light is the main signal that keeps the body clock in step with the day, and its timing, intensity, duration and colour all affect the clock.3 Evening screen light can delay it: reading on a light-emitting e-reader before bed delayed sleep and the body clock compared with reading a printed book.6
- Eating earlier in the day may suit the body's metabolic rhythms. In a five-week proof-of-concept trial in men with prediabetes, eating within a 6-hour window ending before 3 p.m. improved insulin sensitivity and blood pressure even though their weight did not change.4 Larger trials are needed.
Most cells in the body, from neurons to liver and heart cells, contain a molecular clock made of interlocking feedback loops that produce roughly 24-hour rhythms in gene activity and metabolism. A master clock in the brain's suprachiasmatic nucleus (SCN), which receives signals from the eyes, coordinates these peripheral clocks.1
When daily habits fit the clock, eating in daylight hours and sleeping at night, the body's rhythms stay aligned. Chronic misalignment, such as regular late-night eating, irregular light exposure or sleeping at the wrong biological time, has been linked to metabolic problems, although much of that evidence comes from observational studies.
The Molecular Clock: A Brief Overview
The molecular clock is built from feedback loops. The proteins CLOCK and BMAL1 switch on the Period (PER) and Cryptochrome (CRY) genes; PER and CRY proteins build up, return to the nucleus and switch CLOCK-BMAL1 off, completing a cycle of roughly 24 hours. A second loop, involving REV-ERB and ROR, regulates BMAL1. Together these loops drive daily rhythms in thousands of genes.2
The clock's genetic targets are pervasive and closely linked to metabolism, cell growth and physiology.2 One practical consequence is that the time of day a drug is taken can change its effects.
Light: The Master Entrainer
Light is the dominant time cue, or zeitgeber. Light-sensitive cells in the retina that contain the pigment melanopsin, most sensitive to blue light, signal directly to the master clock. Morning light helps set the clock for the day, while light at night suppresses melatonin; the timing, intensity, duration and wavelength of light all matter.3
Evening light delays the clock. In a controlled study, people who read on a light-emitting e-reader in the hours before bed took longer to fall asleep and had less melatonin, a later body clock and lower alertness the next morning than when they read a printed book.6 Practical steps: get outdoor light in the morning, and dim screens and lights in the evening.
Meal Timing and Circadian Metabolism
Eating early in the day is meant to match the body's daily rhythms in metabolism, such as insulin sensitivity, which is the idea behind early time-restricted eating.4
The first trial of this approach, by Sutton and colleagues in 2018, was a supervised feeding study in men with prediabetes. Participants ate within a 6-hour window ending before 3 p.m. or over 12 hours for 5 weeks, then switched schedules, while eating enough to keep their weight stable. The early schedule improved insulin sensitivity, beta-cell responsiveness, blood pressure, oxidative stress and appetite, showing that some benefits of intermittent fasting do not depend on weight loss or caloric restriction.4 As a proof-of-concept study, it needs confirming in larger trials.
Social Jet Lag: The Modern Circadian Crisis
Social jet lag is the mismatch between the sleep timing your body clock prefers (your chronotype) and the timing your work or school schedule demands. Someone whose body prefers to sleep from midnight to 8 a.m. but must rise at 6 a.m. on workdays lives with about 2 hours of social jet lag, much like crossing two time zones every week. In a large epidemiological study, social jet lag was associated with a higher body mass index, independent of sleep duration.5
Sleeping in at weekends recovers some lost sleep but can shift the body clock later. Keeping a similar wake time every day helps keep the clock aligned.
References
- 1Mohawk JA, et al. "Central and peripheral circadian clocks in mammals." Annu Rev Neurosci. 2012;35:445-62. PubMed · DOI
- 2Takahashi JS. "Transcriptional architecture of the mammalian circadian clock." Nat Rev Genet. 2017;18(3):164-179. PubMed · DOI
- 3Duffy JF, Czeisler CA. "Effect of Light on Human Circadian Physiology." Sleep Med Clin. 2009;4(2):165-177. PubMed · DOI
- 4Sutton EF, et al. "Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes." Cell Metab. 2018;27(6):1212-1221.e3. PubMed · DOI
- 5Roenneberg T, et al. "Social jetlag and obesity." Curr Biol. 2012;22(10):939-43. PubMed · DOI
- 6Chang AM, et al. "Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness." Proc Natl Acad Sci U S A. 2015;112(4):1232-7. PubMed · DOI
