Every morning, around 4-5am, our core body temperature is at its lowest. Our temperature rises during the day and reaches a maximum in the early evening around 6-8pm. Afterwards, our temperature decreases. This temperature fluctuation occurs in all humans and is not affected by feeding, rest-activity, light-dark cycles or sleep-wake schedules. Internal clock genes in the brain cause this automatic cycling of temperature over about a 24-hour period and it is an example of a circadian rhythm. The temperature circadian rhythm appears in babies around two months of age.
Every morning around 4-5am, our core temperature is at its lowest. Our temperature rises during the day and reaches a maximum around 6-8pm. Afterwards, our temperature decreases. This temperature fluctuation occurs in all humans and is not affected by feeding, rest-activity, light-dark cycles, or sleep-wake habits. Internal clock genes in the brain cause this automatic cycling of temperature over about a 24-hour period and it is an example of a circadian rhythm. The temperature circadian rhythm appears in babies around two months of age.
Rapid eye movement sleep (REM sleep) has a circadian rhythm component with shorter bouts in the habitual early evening and longer bouts in the habitual later evening that are maintained even with experimentally caused abnormal schedules of feeding, rest-activity, light-dark cycles, or sleep-wake habits.
Every morning, shortly after the minimum body temperature, there are high values of sleep spindle activity in non-REM sleep representing a circadian sleep rhythm component that is independent of feeding, rest-activity, light-dark, or sleep-wake habits.
‘Entrainment’ is defined as a temporal locking process in which one system’s signal frequency entrains the frequency of another system. For example, core body temperature entrains some sleep rhythms. This process is a universal phenomenon that is observed in all humans.
In adults, to get high quality healthy sleep, the night sleep episode should begin in the evening shortly after the maximum core body temperature; that is, when the core body temperature begins to decrease. When the sleep episode occurs when the brain output is in sleep mode, optimum restorative sleep occurs. Sleep quality is highest when it is in synchrony with the body’s biological sleep entrainment.
Studies of adult night shift workers show that chronic abnormal sleep schedules are often associated with adverse health consequences (Blog Posts 1–5). The night shift worker might easily fall asleep during the day after the shift ends, but because the brain is in awake mode, the night shift worker cannot maintain a long duration of sleep. When a sleep episode occurs at the time when the brain output is in awake mode, impaired sleep quality occurs (for example, jet-lag syndrome). This is the opposite of sleep being in synchrony with the body’s biological sleep entrainment. Let’s call it a train wreck.
We do know that children who have late bedtimes (Blog Post 70), who are living in cultures with habitual late bedtimes, are at risk for obesity (Blog Post 81) and other adverse outcomes (Blog Post 91). It is easier for a child to fall asleep and stay asleep (fewer night awakenings) if the sleep period occurs in synchrony with the child’s sleep entrainment (Blog Post 8 and 62). Because of the brain’s immaturity in babies and infants, watching for drowsy signs (Blog Post 9) that signal the brain entering its sleep output mode is better than watching clock time.
We do not know if children who have late bedtimes, who are living in cultures with habitually late bedtimes, have adverse consequences, but in one study, maternal sleep quality improved and maternal postnatal depression decreased when the child’s bedtime was moved earlier (Blog Post 111). The infants had longer night sleep durations of about 1.5 hours and the longest consolidated sleep bout increased by about 1 hour because of fewer signaled awakenings.
