Our bodies run on natural rhythms that regulate many biological functions. One of the most important is the sleep–wake cycle, which follows a circadian rhythm.
Biological Rhythms and the Sleep–Wake Cycle
A biological clock is an innate timing mechanism that controls the cycles of biological rhythms.
The sleep–wake cycle is endogenous (originates within us), but it can be influenced by exogenous cues (signals from the environment), such as light and temperature.
Circadian Rhythms
A circadian rhythm is a biological rhythm lasting about 24 hours.
Examples:
- Sleep–wake cycle – our daily pattern of sleeping and waking.
- Body temperature – highest around 4:30 pm, lowest around 4:30 am.
- Hormone release – melatonin is released in darkness to promote sleepiness, and cortisol is released in the early morning to promote alertness.
Ultradian Rhythms
An ultradian rhythm is a biological rhythm shorter than 24 hours.
Examples:
- Heartbeat and breathing cycles.
- In sleep: the REM–NREM cycle, which repeats roughly every 90 minutes. The exact length can vary depending on factors such as age, health, and recent sleep quality.
The Role of the Suprachiasmatic Nucleus (SCN)
The SCN is an area of the hypothalamus in the brain that receives information about light levels from the eyes.
When light is detected, the SCN sends signals to the pineal gland to adjust the release of melatonin, a hormone that regulates alertness and drowsiness.
Melatonin and Sleep
- Melatonin is produced by the pineal gland and travels through the bloodstream to the whole body.
- High melatonin levels = greater drowsiness.
- Low melatonin levels = greater alertness.
- Less light or darkness → increase in melatonin.
- More light → decrease in melatonin.
This process helps align the sleep–wake cycle with the light–dark cycle of the environment.
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