Sleep science: how sleep works and what shapes it

How to read this page

Each summary restates what the guide found and says no more than it. None of them tells you what to do about your own sleep. Every guide lists its sources, the date we read them and a section on what is uncertain. Where a figure comes from a consensus panel, the guide says so.

Last reviewed September 20, 2026.

The basics

How does sleep work?

Sleep runs in cycles of REM and non-REM stages. NHLBI gives 80 to 100 minutes for a cycle and four to six cycles a night. Two systems help time sleep: a need for sleep that builds while you are awake, and a body clock that follows a rhythm of about 24 hours. Slow-wave sleep peaks in early childhood, drops sharply in the teenage years and keeps decreasing through adulthood.

How much sleep do adults need?

The CDC lists 7 or more hours for ages 18 to 60, and NHLBI and the National Sleep Foundation panel give 7 to 9 hours for adults. NHLBI says adults who sleep less than 7 hours may have more health issues, and that more than 9 hours is not necessarily harmful. These are consensus recommendations from expert panels, and the numbers differ a little between sources.

What times and drives sleep

What is sleep pressure?

Sleep pressure, the tendency to sleep that builds while you are awake, is a well-described concept. Adenosine is widely accepted as one of the substances involved, and caffeine blocks its receptors. We found no human study showing that any of the ten ingredients in one popular sleep formula raises adenosine. The claim that a broken sleep pressure system can be "reset" is an idea, not an established finding.

What is the circadian rhythm?

The circadian rhythm is the set of changes your body goes through over about 24 hours, and light and dark have the biggest influence on it. In a 1999 laboratory study the clock’s intrinsic period averaged 24.18 hours. When you get light matters, and melatonin is the hormone the master clock controls. The light levels in the guide come from an expert consensus for healthy adults, built on laboratory work.

Cortisol and sleep

Cortisol follows a daily rhythm, with most of it released around morning awakening in healthy people. A meta-analysis found moderately higher cortisol on average in people with insomnia. That is an association and does not show which way the cause runs. We found no study showing that a cortisol spike wakes people at 3 a.m.

Evening factors

Body temperature and sleep

Core body temperature falls during sleep. A 2019 meta-analysis associated a warm shower or bath at 40 to 42.5 °C with better self-rated sleep quality and sleep efficiency, and, taken 1 to 2 hours before bed, with falling asleep sooner. Its authors say more research is needed. A 2024 review found that higher temperatures are generally associated with worse sleep.

Blue light and sleep

NHLBI says bright artificial light in the late evening can disrupt melatonin release. Reviews of blue-light glasses disagree. Studies of screen time find associations with somewhat shorter sleep, and they measure hours of use, not light.

Related guides

If your question is about a specific problem, start with waking up at night, waking up at 3 a.m. or racing thoughts at night. For a specific ingredient, see the overview of natural sleep supplements.

Our interest

Two of these guides, on sleep pressure and on cortisol, respond to claims made for Yu Sleep, a liquid supplement we review. Neither contains an affiliate link. Our guides on Yu Sleep do. The affiliate disclosure has the full terms.

Before you rely on any of this

We are an editorial team, not doctors, and nothing here is medical advice. If sleep trouble is persistent, or you have a medical condition or take any medication, talk to a healthcare provider.