Circadian Rhythms Throughout Lifetime

Sleep is vital for healthy human functioning. Circadian rhythms are endogenous cycles in an organism that maintain the sleep-wake cycles in an approximate cycle of 24 hours. It has been shown that in humans, the circadian mechanisms govern not only physiological processes but also psychological and cognitive factors.

However, it is apparent that the individual circadian rhythms are not static throughout the life span. They shift with development, maturity, and aging. Let's explore what affects the circadian rhythm throughout our lives.

The architecture of sleep

Before we move into the differences in sleep between various age groups, it is important to understand the basic architecture of sleep. “Architecture of sleep” is a term used in academic literature and refers to the basic structural organization of normal sleep. Broadly, it is divided into two types of sleep:

• Non-rapid eye movement (NREM).
• Rapid eye movement (REM) sleep.

NREM sleep is further subdivided into 4 stages. Sleep, and its stages, can be viewed as a continuum. During sleep, these stages appear in repeating periods, where one sleep cycle lasts approximately 90 minutes (NREM and REM). During the night, these cycles occur anywhere from 4 to 6 times in adults.

Circadian rhythms of infants and babies

There is a limited body of literature addressing sleep and its architecture in infants, primarily due to ethical approvals and difficulties in obtaining high-quality data. However, it is established that there are many changes that infants undergo in terms of their sleep in their first year of life. It is apparent that the circadian rhythms develop during this period as well, probably by around 4 months of age.

In comparison to other ages, babies tend to sleep significantly more throughout the day. Research indicates that infants spend approximately 17 hours asleep, which is not constrained to a single episode – it is distributed throughout the day and night. As soon as 1 year after birth, the baby's sleep becomes more consolidated to nighttime and shorter by 2.5 hours.

Newborn sleep is broadly categorized into active and quiet sleep. During active sleep babies move, groan, and breathe irregularly, they can even open their eyes, and cry out. Quiet sleep refers to periods of relative stillness and more regular breathing. It is now known that full-term infants spend approximately 50% of their sleep in active sleep; by 6 months of age, this reduces to 25%.

Circadian rhythms of adolescents

There are many hormonal, developmental, and psychological changes that take place during the adolescent years. Research suggests that quality sleep is extremely important during this stage of life. It is noted that the prevalence of late phenotype, i.e., being a night owl, is much higher than in other age groups.

Research suggests that sleep pressure builds up slower in adolescents, together with a slight delay in melatonin production. In comparison to adults, a slightly longer total sleep time is recommended, between 8 and 10 hours. Almost a third of teenagers do not achieve this sleep time, however. The primary reason for this is early school start times.

Circadian rhythms of adults

In order to maintain optimal health, cognitive functioning, and psychological well-being, the American Academy of Sleep Medicine (AASM) and Sleep Research Society (SRS) recommend adults sleep at least 7 hours each night. Research indicates that there are some individual differences in optimal sleep duration and timing.

Nowadays, many adults can more freely choose their schedule and adjust their social obligations in a way that aligns with their preferred circadian rhythm. The “easy rule of thumb” is that as long as an individual feels rested after the night's sleep, is energetic throughout the day, and maintains efficient cognitive functioning, there is no need to worry about their sleep. However, if the sleep schedule and duration are inconsistent, or the total sleep time exceeds 9 hours every day, it is recommended to check in with your physician.

Circadian rhythms of elderly

There are a number of health implications due to increasing age. Unfortunately, sleep is not an exception. Even in the case of healthy aging, the estimates are that as many as 50% of older adults experience difficulties falling asleep, maintaining sleep, and with overall sleep quality. The elderly population has been shown to suffer from more sleep fragmentation and earlier awakening hours.

It is suggested that these disruptions occur primarily by the deterioration of the “master clock” – the suprachiasmatic nucleus (SCN). The neuronal changes in the SCN affect learning and memory, in addition to their impact on sleep and circadian rhythms. With increasing age, the circadian rhythms shift towards the early chronotype, with early bed and wake-up times.