In short: your body has an internal clock of about 24 hours, set mainly by light, that tells you when to feel sleepy and when to feel alert. Adults need at least 7 hours of sleep a night. Regular times, morning daylight and a dark, cool bedroom keep the clock on track.
At a glance
| Sleep adults need | At least 7 hours a night (teenagers 8–10, children 9–12) |
|---|---|
| Body clock | About 24 hours, set mainly by light reaching the eyes |
| Sleep cycle | About 90 minutes: light sleep, deep sleep, REM, repeated 4–6 times |
| Biggest disruptors | Irregular times, bright screens late, caffeine, alcohol, shift work |
| Best habits | Same wake-up time daily, morning daylight, dark cool bedroom |
Light sets the clock
Morning light moves it earlier, evening light later.
Melatonin rises at dusk
Darkness tells the brain it is night; bright light delays it.
Track how you sleep
Note your sleep each morning and see it next to stress and symptoms.
What is the circadian rhythm?
It is the body's internal 24-hour cycle, run by a master clock in the brain (the suprachiasmatic nucleus) that keeps the clocks in your organs in step. It times sleepiness, alertness, body temperature, hormones such as cortisol and melatonin, and even digestion.
The clock runs slightly longer or shorter than 24 hours, so it needs resetting every day. The strongest signal is light reaching the eyes; meals, exercise and social routines help too.
How much sleep do adults need?
At least 7 hours a night for adults aged 18–60, according to the American Academy of Sleep Medicine and the US CDC. Regularly sleeping less is linked to higher risk of obesity, type 2 diabetes, high blood pressure, heart disease, depression and accidents. Teenagers need 8–10 hours, school-age children 9–12.
What happens during sleep?
Sleep runs in cycles of about 90 minutes, each with light sleep, deep (slow-wave) sleep and REM sleep, when most vivid dreams happen. Deep sleep dominates the first half of the night and helps physical recovery; REM grows towards morning and supports memory and mood. Waking at the end of a cycle usually feels easier, which is what the free sleep calculator linked below helps with.
What disrupts the body clock?
Anything that sends the wrong time signal. Bright screens and indoor light late in the evening delay melatonin. Shift work and jet lag force sleep at the wrong biological time. Caffeine blocks the brain's sleep pressure signal for hours; alcohol fragments sleep in the second half of the night. Big weekend shifts in sleep times create a mini jet lag every Monday.
Which habits help you sleep better?
Small, regular habits work best:
- Get up at the same time every day, including weekends.
- Get outdoor daylight within an hour of waking.
- Keep the bedroom dark, quiet and cool, around 18 °C.
- Stop caffeine about 8 hours before bed and keep alcohol low.
- Dim lights and screens in the last hour; keep a wind-down routine.
- If you cannot sleep after about 20 minutes, get up and do something calm until sleepy.
For long-term insomnia, cognitive behavioural therapy for insomnia (CBT-I) is the first-line treatment recommended by sleep medicine guidelines. Talk to a doctor before taking any sleep aid.
When should you see a doctor about sleep?
If you snore loudly, stop breathing or gasp at night, fall asleep during the day without meaning to, or have trouble sleeping three nights a week for three months or more. These can be signs of sleep apnoea, insomnia or another sleep disorder that needs proper assessment.
Key terms
- Circadian rhythm: the body's internal cycle of about 24 hours that times sleep, hormones and temperature.
- Melatonin: a hormone released in darkness that signals night to the body.
- Sleep cycle: a sequence of light, deep and REM sleep lasting about 90 minutes.
- REM sleep: the stage with rapid eye movements and vivid dreams, important for memory and mood.
- Chronotype: your natural tendency to be a morning or an evening person.
- Sleep hygiene: everyday habits and conditions that support good sleep.
Sources
- NIH, National Institute of General Medical Sciences: Circadian rhythms
- NHLBI: Sleep deprivation and deficiency
- CDC: About sleep
- Watson et al., AASM and SRS consensus on adult sleep duration (2015)
Frequently asked questions
Is 6 hours of sleep enough?
For most adults, no. Guidelines recommend at least 7 hours. A small number of people genuinely need less, but regularly sleeping 6 hours or less is linked to poorer health and slower reactions.
Why do I wake up at the same time every night?
Brief awakenings between sleep cycles are normal; most are not remembered. Waking at the same time can come from a regular cycle pattern, noise, light, a full bladder, stress or alcohol. If it leaves you tired every day, talk to a doctor.
Does catching up on sleep at the weekend work?
Partly. Extra sleep repays some debt, but big shifts in sleep times confuse the body clock. Keeping the wake-up time steady and going to bed earlier works better.
Are evening people lazy?
No. Chronotype is partly genetic. Evening types have a later body clock and struggle with early schedules; morning light and consistent times can shift it gently.
How can I reset my body clock after jet lag?
Move gradually to the new time zone: seek daylight in the local morning, avoid bright light in the local evening, and keep meals and sleep on local time. The clock shifts by about an hour a day.
Does my phone's blue light really affect sleep?
Bright light of any colour in the evening delays melatonin; blue-rich screen light does so most. Dimming screens, using night mode and stopping in the last hour before bed help.
Try it in Anpheros Daily
Note how you sleep, eat, feel and move in Anpheros Daily, next to your lab results, and see the patterns over weeks.
Anpheros builds a medical record that the patient controls and can share with any clinic, on an HL7 FHIR R4 platform, based in the EU. These guides help you understand and organise your health; they do not replace your doctor.
About Anpheros →
AnpherosThe more you log, the clearer your record
Your health history in one place, under your control.
Free · Works in your browser · No card
