The circadian rhythm is the body's internal 24-hour biological clock that regulates cycles of wakefulness and sleep in response to environmental light and darkness.
The circadian rhythm is the body's internal 24-hour clock, governing cycles of alertness and sleepiness in response to environmental cues, primarily light and darkness. It regulates not only the sleep-wake cycle but also body temperature, hormone release, digestion, and other bodily functions. In humans, the master circadian clock is located in a region of the brain called the suprachiasmatic nucleus (SCN).
How does the circadian system work?
As evening approaches and light levels fall, the pineal gland produces melatonin — a hormone the NHS describes as being "released by the body in the evening to let your brain know it's time to sleep." Melatonin levels rise through the night and return to low levels during the day. This daily rhythm of melatonin production helps regulate when a person falls asleep and wakes. Morning light suppresses melatonin and promotes wakefulness. Other cues that help synchronise the body clock — known as zeitgebers — include meal timing, exercise, and social routines.
When do infants develop a circadian rhythm?
Newborns do not have an established circadian rhythm at birth. According to a review hosted by the US National Institutes of Health (NIH), the circadian system begins to organise postnatally: production of melatonin, described as "critical to the permanent establishment of circadian rhythms," begins to emerge at around three months of age, while cortisol rhythms develop between approximately eight weeks and nine months. Before this, newborns cycle through sleep and waking in relatively short, irregular bursts unrelated to the time of day. The NHS notes that newborn babies "may sleep for anywhere between eight to 18 hours a day, but only for two to three hours at a time."
What does this mean for infant sleep at night?
The absence of a mature circadian rhythm in the first months of life is a key reason why newborns do not distinguish day from night and wake frequently around the clock. The Lullaby Trust notes that babies have small stomachs and will wake often throughout the night to feed. As the circadian rhythm matures over the first few months, sleep gradually becomes more consolidated during the night.
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