Circadian glossary

A chemical that builds up in the brain the longer you are awake and makes you feel sleepy. Caffeine increases alertness by blocking adenosine from reaching its receptors.

A circadian pattern where sleep occurs earlier than typical social schedules.

Another name for the circadian clock, also called the body clock.

Short-wavelength light that the circadian system is most sensitive to. It strongly suppresses melatonin and shifts the clock, but other wavelengths, including violet and green, reset the clock too.

Glasses that filter blue light. Because other wavelengths also reset the circadian clock, blocking only blue light does just part of the job – the timing of all light exposure matters more.

A treatment that uses timed exposure to bright light, often from a light therapy lamp, to shift or stabilize the circadian rhythm.

A stimulant that reduces sleepiness by blocking adenosine. Small, well-timed doses help maintain alertness, but caffeine taken even early in the day can affect sleep that night.

The scientific field that studies biological timing systems, including circadian rhythms and sleep-wake cycles.

Timing medical treatments, such as medication, to the body's circadian rhythms to improve how well they work or reduce side effects.

An individual's natural preference for sleeping earlier or later, commonly referred to as an "early bird" or a "night owl."

The process of shifting your circadian clock to a new time zone or work schedule. Without help it adjusts about an hour a day; correctly timed light, light avoidance and melatonin can speed it up.

The daily rise and fall in alertness driven by the circadian clock – typically lowest in the early morning hours and highest in the late morning and early evening.

When the circadian clock is synchronized with sleep schedules, light exposure, and daily activities.

Designing shift schedules, rotations and routines around how the circadian clock works – for example the direction and speed of rotations – to reduce fatigue and health risks.

The internal biological timing system that coordinates circadian rhythms and aligns the body with the day-night cycle.

Chronic disturbance of the circadian system that can negatively impact sleep, health, and performance.

Tiredness and reduced alertness that come from being awake at the wrong circadian time, such as during a night shift or after crossing time zones, rather than from lack of sleep alone.

Maintaining a stable circadian rhythm that supports sleep, metabolism, mood, and cognitive performance.

When the circadian clock is out of sync with sleep timing or environmental cues, often due to shift work or jet lag.

Timing sleep, light, meals, caffeine and activity to your circadian clock so you are alert and performing well when it matters.

Daily fluctuations in cognitive and physical performance driven by the circadian clock.

A specific point in the circadian rhythm cycle, such as biological morning, afternoon, evening, or night.

A graph showing how light at different times of the biological day shifts the circadian clock: light before the crossover point (around your lowest body temperature) delays the clock, light after it advances the clock.

A group of sleep-wake conditions in which the timing of the circadian clock is persistently out of step with the desired or required sleep schedule. They are diagnosed and treated by healthcare professionals.

The natural ~24-hour biological cycle that regulates sleep, alertness, hormones, metabolism, body temperature, and many other physiological processes.

When processes such as sleep, hormone release, digestion and body temperature happen within your 24-hour biological day.

The daily rise and fall of core body temperature, which is lowest in the early morning and highest in the late afternoon or evening. Its low point is a key marker of circadian timing.

The daily cycle of cortisol levels that typically peak in the morning and decline throughout the day.

The practice of moving clocks forward one hour in spring and back one hour in autumn. The one-hour shift in social time can temporarily misalign sleep with your circadian clock, especially in spring.

A circadian pattern where sleep occurs later than typical social schedules. Also called delayed sleep phase syndrome (DSPS).

The time in the evening when melatonin starts to rise under dim light – the most widely used marker of where a person's circadian clock is.

A person whose circadian rhythm naturally favors early sleep and wake times, also called a morning person.

The process by which circadian rhythms synchronize with environmental signals such as light.

The drive for sleep that builds the longer you stay awake – partly through adenosine – and is released during sleep.

Biological rhythms longer than 24 hours, such as menstrual cycles.

Persistent difficulty falling asleep, staying asleep, or waking too early despite having the opportunity to sleep. Sleeping at the wrong circadian time can cause similar problems.

Specialized cells in the eye that detect light and send signals to the circadian clock.

Jet lag is a temporary physical and mental condition caused by a misalignment between your circadian rhythms and a new time zone after crossing time zones faster than your body’s circadian clocks can adapt.

The daily pattern of daylight and darkness that helps synchronize circadian rhythms.

The timing and intensity of light reaching the eyes, which strongly influences the circadian clock.

Excess artificial light at night, outdoors and indoors, which can suppress melatonin and disrupt circadian timing.

How strongly the circadian clock responds to light, depending on its color, brightness, duration, timing, and your recent light history.

A light-sensitive photopigment in specialized retinal cells that signals light exposure to the circadian clock.

A hormone produced by the pineal gland during your biological night that signals darkness to the body. Taken as a supplement at the right time, it can help shift the circadian clock – it is not a sleeping pill.

Reduction of melatonin production caused by exposure to light at night.

A brief, involuntary lapse into sleep lasting from a fraction of a second to several seconds, often without the person noticing. Microsleeps are a serious safety risk when driving or working at night.

A person whose circadian rhythm naturally favors later sleep and wake times.

A work shift that covers all or most of the night – the time your circadian clock is promoting sleep and alertness is at its lowest.

Moving the circadian clock earlier, so you get sleepy and wake up earlier. It is what your body needs to do when you fly east.

Moving the circadian clock later, so you get sleepy and wake up later. It is what your body needs to do when you fly west.

A move of the circadian clock to an earlier time (phase advance) or a later time (phase delay), mainly caused by light at specific times of the biological day.

A short nap, typically 10–20 minutes, that can boost alertness while limiting grogginess on waking. Well-timed naps are a useful tool for shift workers and travelers.

An overnight flight that departs late in the evening and arrives early the next morning, often cutting sleep short on top of the time zone change.

Rapid eye movement sleep, the sleep stage associated with vivid dreaming, when the brain is highly active while most muscles are temporarily relaxed. REM sleep is most abundant in the later part of the night.

The SCN is the master circadian clock in the hypothalamus, keeping a ~24-hour rhythm synchronized to the light-dark cycle via the eyes.

A form of depression with a seasonal pattern, usually in autumn and winter when days are short. It is diagnosed and treated by healthcare professionals, sometimes with bright light therapy.

Work hours that fall substantially outside the conventional daytime window, including night, early-morning, evening and rotating schedules.

The disruption to sleep, alertness and health that happens when work hours force you to be awake and asleep at the wrong circadian times.

A shift worker is anyone whose work hours fall substantially outside the conventional daytime window (roughly 07:00–18:00), including night, early-morning, evening, or rotating schedules.

A sleep disorder in which breathing repeatedly stops and starts during sleep, fragmenting sleep and causing daytime sleepiness. It requires diagnosis and treatment by a healthcare professional.

The accumulated shortfall between the sleep you need and the sleep you get. It builds over consecutive nights of short sleep and impairs alertness, mood, and performance.

Getting less sleep than you need, either for one night or over a longer period. It impairs attention, reaction time, mood, metabolism, and immune function.

Everyday habits and bedroom conditions that support good sleep, such as a regular schedule, a dark and quiet bedroom, and avoiding caffeine and alcohol close to bedtime.

The grogginess and reduced alertness and performance you feel right after waking. It can take time to fully wear off, especially when waking during your biological night.

Moving your sleep and wake times gradually earlier or later – for example before a trip – to start adjusting your circadian clock in advance. It only helps when light exposure is shifted at the same time.

The daily rhythm of sleep and wakefulness regulated by circadian timing and sleep pressure.

Circadian disruption caused by differences between weekday and weekend sleep schedules. Also written social jetlag.

Limiting daily food intake to a consistent window of hours. Food is a weak time cue for the clock in the brain, and there is no accepted method for using eating or fasting to reset circadian rhythms.

The most-downloaded jet lag app in the world (more than 1.8 million users). It's based on the latest sleep and circadian neuroscience.

The #1 app for shift workers. It's based on the latest sleep and circadian neuroscience.

Biological cycles shorter than 24 hours, such as the stages of sleep.

An environmental cue that resets or synchronizes the circadian clock, with light being the most powerful.