Polarisbioseditoryour inner clock: a guide to body rhythms, light and sleep

An independent guide to biological rhythms, light and sleep. We do not diagnose or replace a doctor, and we are not affiliated with the previous website on this domain. If a life is in danger, call 112 (or your local emergency number).

Glossary

Glossary

Words about body rhythms come up more and more in everyday conversation: someone calls themselves a “night owl,” someone else complains about jet lag after a vacation. This glossary explains the key terms in plain language. It helps you understand articles and conversations about sleep, but it is not meant for assessing your own condition: that's a doctor's job.

Knowing the word “chronotype” is useful. But only a doctor who has heard your whole story can put an accurate name to your problem.

— Editor's note

Rhythms and cues

Circadian rhythm
The body's internal rhythm with a period of about one day. It affects sleep, wakefulness and other processes and adjusts to external cues, the main one being light. In 2017, the Nobel Prize in Physiology or Medicine was awarded for discoveries of molecular mechanisms controlling this rhythm.
Inner clock
An everyday name for the system that sets the body's daily rhythm. Morning light helps it stay in sync with the outside day, while bright light late in the evening can push it later.
Melatonin
A hormone the body produces mainly during the dark hours. Any questions about melatonin as a supplement are for your doctor only.
Chronotype
A person's tendency toward earlier or later sleep and wakefulness — what people call “early birds” and “night owls.” Chronotype can shift with age: teens tend to lean toward later hours, older adults toward earlier ones.

Routine and rhythm disruption

Circadian misalignment, jet lag
A mismatch between your inner clock and external time, for example after flying across several time zones. It usually shows up as trouble sleeping and daytime sluggishness until the body adjusts.
Social jet lag
The difference between your routine on workdays and days off: on weekdays you get up early to an alarm, and on weekends you sleep much longer. It's as if you flew back and forth across time zones every week.
Shift work
Work outside normal daytime hours: at night, early in the morning or on a rotating schedule. It requires you to be awake when your inner clock is preparing the body for sleep, so sleep and light need special planning.
Polar night
The time of year above the Arctic Circle when the sun doesn't rise above the horizon. There is little daylight, and a fixed wake-up time and bright light at home in the morning help maintain a routine.
Polar day
The time of year above the Arctic Circle when the sun doesn't set. It's light around the clock, so a darkened bedroom and a consistent bedtime are especially important for sleep.

Help and tracking

Sleep hygiene
A set of everyday habits that support healthy sleep: a consistent wake-up time, a dark and quiet bedroom, a calm evening, no coffee late in the day and fewer bright screens before bed.
Sleep diary
Daily notes on when you went to bed, fell asleep, woke up during the night and got up, plus how your day went. It helps a doctor see your routine more clearly than memory alone.
Sleep specialist
A doctor who focuses on sleep disorders. People usually see one on the advice of a primary care doctor or neurologist; ask your own doctor or clinic how this works where you live.

Where these terms appear in the guide

  • Topics

    A map of topics: how the inner clock works, light through the day, chronotype, shift schedules, long flights, northern latitudes, plus the catalogue.

  • Prevention

    Everyday habits for a steady rhythm: a consistent wake-up time, morning light, movement, a quiet evening and seeing a doctor in good time.

  • Approaches

    How to plan sleep and light around shift work, after a flight and above the Arctic Circle, who to see and how to keep a sleep diary.

  • Research

    How to read news about sleep and body rhythms: bold headlines, mouse vs. human studies, surveys vs. experiments, and who paid for it.

  • Ask your doctor

    Questions about sleep, shift schedules, daytime sleepiness and supplements that are worth raising with your own doctor.

  • Topics

    A map of topics: how the inner clock works, light through the day, chronotype, shift schedules, long flights, northern latitudes, plus the catalogue.

  • Approaches

    How to plan sleep and light around shift work, after a flight and above the Arctic Circle, who to see and how to keep a sleep diary.

  • Ask your doctor

    Questions about sleep, shift schedules, daytime sleepiness and supplements that are worth raising with your own doctor.

  • Sources

    What sources this guide relies on, why it contains so few numbers and how to check advice about sleep and light yourself.

Sources

  • Nobel Committee — the 2017 Nobel Prize in Physiology or Medicine
  • American Academy of Sleep Medicine — recommendations on adult sleep duration (2015)

This guide draws on the Nobel Committee's announcement of the 2017 prize, the recommendations on adult sleep duration and the WHO guidelines on physical activity as they were available when the texts were written. Scientific understanding of sleep rhythms keeps evolving, so questions about your own health are best discussed with a doctor.