The biological preparation for sleep begins long before we close our eyes.

Hours beforehand, numerous processes change in the background. Body temperature, alertness, and hormone signals prepare our bodies for the transition between day and night.

Melatonin plays a central role in this process. This hormone, produced primarily in the pineal gland, follows a pronounced 24-hour rhythm. Its concentration is low during the day. As darkness increases, it rises in the evening, reaches higher levels during the night, and then falls again towards morning.

Melatonin is therefore often referred to as the "sleep hormone". In fact, however, this term only describes part of its biological significance.

Melatonin doesn't simply make us tired. Its increase is part of a complex timing system that our body uses to distinguish between day and night.

This interplay becomes particularly interesting when falling asleep is difficult: What happens when the natural melatonin signal is altered by our light environment? And what difference can the timing and amount of additional melatonin intake make?

When light controls melatonin release

The most important external pacemaker for our internal clock is light.

Special light-sensitive cells in the retina detect the brightness of our surroundings and transmit this information to the suprachiasmatic nucleus, or SCN for short – the central control point of our internal clock.

As the light decreases in the evening, the signal transmission to the pineal gland changes and the body's own melatonin production increases.

Artificial light has fundamentally altered this natural transition. The sun may have long since set while we are still surrounded by indoor lighting and screens for several more hours.

How sensitive this system is was demonstrated in a human study with 116 healthy adults.[1] In the eight hours before going to sleep, the participants were exposed to either normal room light of less than 200 lux or very weak light of less than 3 lux.

Under ambient light, the melatonin rise began later in 99% of participants. At the same time, the duration of nighttime melatonin production was reduced by approximately 90 minutes.

When participants were exposed to room light even during their usual sleep time, melatonin production was suppressed by more than half in 85% of them.[1]

Light is therefore much more than just brightness for our bodies. It provides biological information about what time of day we are.

Why 10 PM isn't always 10 PM

The evening rise in melatonin is so characteristic that scientists use it to determine the individual position of our internal clock.

The so-called “Dim Light Melatonin Onset”, or DLMO for short, describes the time at which melatonin levels begin to rise measurably under low light conditions in the evening.

This makes visible something that an ordinary clock cannot capture: our biological time.

Because 10 p.m. on the clock doesn't necessarily mean the same biological time for our bodies as it does for someone else's. Our circadian rhythms can vary from person to person.

That's precisely why, when it comes to melatonin, it's not only interesting to know how much is absorbed, but also when.

Why timing matters with melatonin

Scientists investigated how strongly additionally ingested melatonin depends on biological time in 34 healthy adults.[2]

For three consecutive days, participants received 0.5 mg of melatonin at different times relative to their individual biological time. The resulting response curve was then compared with data from a higher dose of 3 mg.

Even 0.5 mg could measurably shift the circadian rhythm. However, the extent of this shift depended significantly on when the melatonin was ingested.[2]

This highlights an important difference: Melatonin is not simply a substance whose significance can be explained solely by its dosage. The timing of its intake is also part of its biological signaling effect.

This observation is complemented by a larger evaluation of clinical studies.

A meta-analysis published in 2024 summarized 26 double-blind, randomized, controlled trials with a total of 1,689 observations.[3]

Across all included studies, participants taking additional melatonin took an average of 9.31 minutes less to fall asleep than those taking a placebo.

At the same time, the total sleep time increased by an average of 19.67 minutes.[3]

The scientists also investigated why the results varied between individual studies. Timing again came into focus: the time interval between taking the medication and sleep was related to the observed changes in sleep onset time.

This connects chronobiology to a very specific topic: the transition from wakefulness to sleep.

Sleep is more than melatonin

As important as melatonin is for the day-night rhythm, sleep cannot be reduced to a single messenger substance.

Sleep pressure, light, the body's internal clock, physical activity, and our daily habits are all interconnected. This is one reason why, in addition to melatonin, various plants are traditionally and scientifically studied in connection with sleep and evening rest.

Among the best-known are valerian and lavender.

A randomized, double-blind, placebo-controlled human study followed 80 adults with sleep problems for eight weeks.[4]

The valerian extract studied showed improvements in several sleep parameters compared to placebo, including time to fall asleep, actual sleep duration and sleep efficiency.[4]

Lavender has also been studied in controlled trials.

A meta-analysis of eleven randomized controlled trials with a total of 628 adults examined interventions using lavender essential oil. The combined results showed improvements in sleep quality.[5]

Melatonin, valerian and lavender are thus being examined from different scientific perspectives in relation to sleep.

From melatonin signal to evening routine

Research on melatonin shows, above all, that falling asleep is not an isolated event that only begins the moment we lie down in bed.

Our bodies begin preparing for the night hours in advance.

Light provides one of the most important external signals. At the same time, every person has an individual biological time that does not necessarily correspond exactly to the clock.

That is precisely why timing is becoming increasingly important in melatonin research.

Human studies show that even relatively small amounts of melatonin can influence the circadian rhythm and that the timing of intake is crucial for this response.[2] At the same time, meta-analyses of controlled studies show measurable changes in sleep onset time and total sleep duration.[3]

This also shifts the view of sleep: away from the idea of ​​a single "sleep hormone" and towards a complex biological transition between day and night.

Anyone who wants to understand why falling asleep is sometimes difficult should therefore not look to the night first. But rather the hours before.

Sources

[1] Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans. Journal of Clinical Endocrinology & Metabolism. 2011;96(3)–E472.

[2] Burgess HJ, Revell VL, Molina TA, Eastman CI. Human Phase Response Curves to Three Days of Daily Melatonin: 0.5 mg Versus 3.0 mg. Journal of Clinical Endocrinology & Metabolism. 2010;95(7):3325–3331.

[3] Cruz-Sanabria F, et al. Optimizing the Time and Dose of Melatonin as a Sleep-Promoting Drug: A Systematic Review of Randomized Controlled Trials and Dose-Response Meta-Analysis. Journal of Pineal Research. 2024;76.

[4] Shinjyo N, et al. Standardized Extract of Valeriana officinalis Improves Overall Sleep Quality in Human Subjects with Sleep Complaints: A Randomized, Double-Blind, Placebo-Controlled, Clinical Study. Advances in Therapy. 2023;40:4209–4225.

[5] Cheong MJ, Kim S, Kim JS, Lee H, Lyu YS, Lee YR, Jeon B. A Systematic Literature Review and Meta-Analysis of the Clinical Effects of Aroma Inhalation Therapy on Sleep Problems. Medicine (Baltimore). 2021;100(9).