Circadian Rhythm, the SCN and Melatonin: Sleep Regulation for the MCAT

The circadian rhythm is a roughly 24-hour internal cycle. The master clock is the suprachiasmatic nucleus (SCN) of the hypothalamus. Light is the strongest cue that sets it.

The pathway

1. Light hits the retina, including intrinsically photosensitive retinal ganglion cells.
2. The retinohypothalamic tract carries the signal to the SCN.
3. The SCN controls the pineal gland, which releases melatonin mainly in darkness.
4. Rising melatonin promotes sleepiness; morning light suppresses it.

External time cues are called zeitgebers. Light is the main one; meals and social schedules also contribute.

Two processes behind sleep

• Circadian drive: the clock times when sleep is favored.
• Sleep pressure: the longer you stay awake, the stronger the drive to sleep. Adenosine buildup is a commonly cited contributor.

Related terms

• Jet lag: the clock is out of step with the new time zone.
• Insomnia: difficulty falling or staying asleep.
• Narcolepsy: sudden sleep attacks, with REM features that intrude into waking.
• Sleep apnea: repeated breathing interruptions during sleep.
• Night terrors and sleepwalking: arise from N3, not REM.

Common traps

• Saying the pineal gland is the master clock. The SCN is the clock; the pineal makes melatonin.
• Thinking melatonin is released by light. Light suppresses it.
• Placing night terrors in REM.

One-minute recap

• SCN is the clock; light is the main zeitgeber.
• Pineal gland releases melatonin in darkness.
• Sleep depends on circadian drive plus sleep pressure.