Circadian rhythms are roughly 24-hour cycles that help living organisms match their internal biology to the day and night cycle. In humans, these rhythms influence sleep, body temperature, hormone release, digestion, alertness, and mood. They matter because good timing helps the body use energy efficiently, repair tissues, and stay mentally focused.
When the clock is disrupted, sleep quality, learning, metabolism, and health can suffer.
The master clock in the brain is the suprachiasmatic nucleus, or SCN, which receives light information from the eyes. Morning light helps reset this clock, while darkness promotes melatonin release from the pineal gland to signal biological night. Inside many cells, clock genes such as CLOCK, BMAL1, PER, and CRY create feedback loops that rise and fall across the day.
Jet lag, shift work, late-night screens, and irregular sleep schedules can push the internal clock out of sync with the outside world.
Key Facts
- A circadian rhythm is an internal biological cycle with a period of about 24 hours.
- The suprachiasmatic nucleus, or SCN, is the brain's master circadian clock.
- Light detected by the retina resets the SCN and helps align the body clock with the environment.
- Melatonin secretion increases in darkness and helps signal that it is time for sleep.
- Core clock gene feedback can be summarized as CLOCK + BMAL1 activate PER and CRY, then PER and CRY inhibit CLOCK and BMAL1.
- Circadian phase shift = internal clock time minus local environmental time, which helps explain jet lag.
Vocabulary
- Circadian rhythm
- A biological cycle that repeats about every 24 hours and regulates daily patterns such as sleep and hormone release.
- Suprachiasmatic nucleus
- A small region of the hypothalamus that acts as the master clock for coordinating circadian rhythms.
- Melatonin
- A hormone released mainly at night that signals darkness and helps prepare the body for sleep.
- Clock genes
- Genes whose protein products form feedback loops that generate daily rhythms inside cells.
- Jet lag
- A temporary mismatch between the body's internal clock and the local day-night schedule after rapid travel across time zones.
Common Mistakes to Avoid
- Thinking circadian rhythms are only about sleep is wrong because the clock also regulates temperature, hormones, digestion, alertness, and metabolism.
- Assuming the body clock instantly adjusts to a new time zone is wrong because circadian phase usually shifts gradually over several days.
- Using bright screens late at night and expecting no effect is wrong because blue-rich light can delay melatonin release and shift sleep timing later.
- Believing melatonin is simply a sleeping pill is wrong because it is mainly a timing signal, and its effect depends strongly on when it is taken.
Practice Questions
- 1 A student normally sleeps from 10:30 p.m. to 6:30 a.m. How many hours of sleep is this, and what fraction of a 24-hour day does it represent?
- 2 After flying 6 time zones east, a traveler's internal clock says 10:00 p.m. when the local time is 4:00 a.m. What is the circadian phase shift in hours?
- 3 Explain why getting bright outdoor light soon after waking can help a person adjust to an earlier sleep schedule.