Sun, moon, phone, glasses and clock — **What sets the internal clock.** Morning sunlight, darkness at night, evening screens and regular sleep times.

How does the internal clock work?

The master clock in the brain is reset every day by light information coming from the eyes. Daylight sets the clock to "daytime"; darkness allows the sleep hormone melatonin to be released. The liver, muscle and fat tissue also have their own clocks, which are set by mealtimes. When these clocks work in step with each other, metabolism also runs smoothly.

Light is a clock, too.

Evening light and melatonin

Melatonin is released in the dark and tells the body that night has come. In a controlled laboratory study, staying in ordinary room light during the hours before bedtime delayed the onset of melatonin release in 99 percent of participants, compared with dim light.

Room light before bedtime shortened the duration of melatonin released during the night by about

0min

The night the body perceives is shortened by light.

Laboratory study in healthy volunteers comparing dim light with room light.
Gooley JJ et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab 2011;96(3):E463-E472. doi:10.1210/jc.2010-2098

Screens and blue light

Phone, tablet and computer screens emit light mainly in the blue wavelength, and the internal clock is most sensitive to this light. People who read from a light-emitting screen in the hours before bed fell asleep later than those who read a printed book, released less melatonin, had their internal clocks shifted later and felt less alert the next morning.

Do blue-light glasses work?

Glasses with a blue-light filter are widely sold to reduce eye strain and improve sleep. A 2023 Cochrane review of 17 randomized studies found no evidence that these glasses reduce screen-related eye strain. Their effect on sleep is unknown, because the studies are few and small.

So the most reliable way to protect yourself from blue light in the evening is not glasses but putting the screen away early and dimming the lights at home. Wearing glasses is not harmful, but they should not be seen as a solution on their own.

When the rhythm is disrupted: growth hormone and insulin resistance

Most of the night-time release of growth hormone happens during deep sleep in the first half of the night. Going to bed late or having interrupted sleep shortens this period.

When the internal clock and the timing of meals and sleep drift apart, metabolism is also disturbed. In a laboratory study, when volunteers ate and slept on a schedule shifted by about 12 hours from their usual one, blood sugar rose by 6 percent even though insulin increased by 22 percent; this indicates that insulin was working less effectively. Leptin, the satiety hormone, fell by 17 percent.

Among healthy volunteers whose rhythm was shifted, in

0/ 8

people, blood sugar after a meal reached the prediabetes range.

10-day laboratory protocol; 8 participants with sufficient data. This is a small study; it gives direction for understanding the effects of shift work.
Scheer FA et al. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proc Natl Acad Sci U S A 2009;106(11):4453-4458. doi:10.1073/pnas.0808180106

Practical tips

Go out into daylight within the first hour after waking up.
Dim the lights at home in the evening; put screens away at least one hour before bed.
Go to bed and get up at the same time every day, including weekends.
Do not leave heavy meals for late in the evening.
Keep the bedroom dark; if you need a night light, make it dim and warm-coloured.
If you work shifts, ask your doctor about regular sleep times and nutrition.

References

Gooley JJ et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab 2011;96(3):E463-E472. doi:10.1210/jc.2010-2098
Chang 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):1232-1237. doi:10.1073/pnas.1418490112
Singh S et al. Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults. Cochrane Database Syst Rev 2023;8:CD013244. doi:10.1002/14651858.CD013244.pub2
Van Cauter E et al. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA 2000;284(7):861-868. doi:10.1001/jama.284.7.861
Scheer FA et al. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proc Natl Acad Sci U S A 2009;106(11):4453-4458. doi:10.1073/pnas.0808180106

This page is for information only; an examination is needed for diagnosis and treatment.

Circadian rhythm