Circadian rhythms and sleep cycles

In this story

• Why it matters
• The master clock
• Sleep runs in cycles, not blocks
• Why some mornings feel heavier
• Your body temperature is moving the whole time
• Try this tonight
• Sources

Sleep is not a switch. Over the course of a night your brain runs a scheduled sequence of changes in hormone levels, body temperature, alertness and sleep depth, and it runs that sequence whether or not you notice. The schedule is set by an internal clock called the circadian rhythm.

Understanding how that clock works is the first step toward a bedroom that supports your body instead of working against it. Most sleep advice treats the night as one long block of time. The biology says otherwise.

Why it matters

Two people can each sleep eight hours and wake up feeling completely different. The difference is often not duration. It can be how much deep sleep they managed early on, how broken the night was, and whether the room let their body cool when it needed to.

If sleep were uniform, a fixed bedroom would be fine. It is not uniform, so a fixed bedroom is a compromise for most of the night.

Every one of those variables moves while you are asleep. A room that holds one setting is only correct for part of the night.

Breaking down the science

The master clock

At the center of the system is the suprachiasmatic nucleus, a small region of the hypothalamus usually described as the body's master clock.^[2] It takes light entering the eyes as its main input and uses it to decide when melatonin is released in the evening and when cortisol begins climbing before morning.

Because light is the primary signal, evening light exposure is not a neutral act. It is an instruction to the clock.

Sleep runs in cycles, not blocks

Sleep is built from repeating cycles rather than one continuous state. A complete cycle takes roughly 90 to 110 minutes and moves through four stages.^[3]

N1 is the short transition out of wakefulness. N2 is stable sleep where heart rate slows and body temperature keeps falling. N3 is deep or slow-wave sleep, the stage most closely tied to physical recovery. REM is rapid eye movement sleep, associated with dreaming, emotional processing and memory consolidation.

The order matters. Deep sleep is front-loaded, which is one reason a late bedtime costs more than the arithmetic suggests. You do not simply lose an hour. You lose an hour from the part of the night doing the heaviest work.

Why some mornings feel heavier

Sleep inertia is the heavy, disoriented feeling just after waking. Waking from deep sleep can make it worse, though studies do not all agree. Prior sleep loss and waking during your biological night add to it.^[4]

Your body temperature is moving the whole time

Core temperature is around 37°C during the day. It begins falling roughly two hours before sleep, drops by about 2°F overnight, reaches its lowest point in the early morning and then climbs again ahead of waking.^[5]

The body sheds that heat by widening blood vessels in the hands and feet, a process called distal vasodilation. If the room is too warm, that mechanism has less room to work, and slow-wave sleep suffers.^[6]

Try this tonight

• Protect the start of the night. A later bedtime cuts into the hours with the most deep sleep.
• Treat evening light as a message to your body clock, and keep it low before bed.
• Keep the room cool, so your body can shed heat as it falls asleep.

This article is general education, not medical advice. If you are worried about your sleep, talk to a doctor.

Sources

1. 1.Institute of Medicine; Colten HR, Altevogt BM, editors (2006). Sleep Physiology, in Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. National Academies Press. Accessed Jul 2026.
2. 2.Bryan L, Sleep Foundation (updated 2025). Circadian Rhythm. Sleep Foundation. Accessed Nov 5, 2026.
3. 3.Patel AK, Reddy V, Araujo JF (n.d.). Physiology, Sleep Stages. StatPearls Publishing. Accessed Jul 2026.
4. 4.Hilditch CJ, McHill AW (2019). Sleep inertia: current insights. Nature and Science of Sleep, 11:155-165. Accessed Nov 5, 2026.
5. 5.Pacheco D, Sleep Foundation (updated 2025). The Best Temperature for Sleep. Sleep Foundation. Accessed Nov 5, 2026.
6. 6.Okamoto-Mizuno K, Mizuno K (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1):14. Accessed Nov 5, 2026.