Something inside that has the ability to keep time, an internal body clock
Exogenous zeitgeber
An environmental cue from the outside world that keeps our internal body clock on time, also called entrainment
Circadian rhythm
A biological rhythm that lasts around 24 hours
The sleep/wake cycle is the most obvious circadian rhythm, but we also have circadian rhythms for releasing hormones and varying body temperature
Sleep-wake cycle
We have a period of sleep and a period of wakefulness every 24 hours
Exogenous zeitgebers (cues like light, clocks, social cues)
Can conflict with the internal body clock, leading to issues like jetlag
Suprachiasmatic nucleus (SCN)
The endogenous pacemaker for the sleep/wake cycle, also known as the master clock
Light detected by the SCN
Suppresses melatonin production in the pineal gland, making you feel less sleepy
The main exogenous zeitgeber for the sleep/wake cycle is the amount of light received in the eyes
Social cues like meal times and environmental cues like clocks also work as exogenous zeitgebers
Free-running circadian rhythm
The endogenous biological rhythm maintains itself without entrainment by external cues
Studies on free-running circadian rhythm
Siffre's 179-day stay in a cave without access to natural light or clocks
More recent studies controlling for artificial light use
Evidence suggests the SCN is the biological structure responsible for the circadian rhythm in the sleep/wake cycle
Disruption to sleep patterns can lead to increased anxiety, decreased alertness, and decreased vigilance
Understanding how the sleep-wake cycle is affected by exogenous zeitgebers can help develop treatments for jetlag, improve performance of athletes who travel, and help shift workers be more productive
Knowing the circadian rhythms like blood pressure can help with timing drug treatments, benefiting health services
Exposure to blue light at night and from personal devices has implications for maintaining healthy sleep
Even with social cues, a man blind from birth had a 24.9 hour circadian rhythm, out of sync with the rest of society
There is an evolutionary explanation for the day/night cycle of humans and other animals specialized for daytime activity
Nocturnal animals like rats have a circadian pacemaker shifted 12 hours, so their wakefulness is at night