The clock is ticking

The body undergoes changes at every stage of life, and many of these changes affect sleep. The circadian rhythm is the internal clock that controls sleep and wake cycles, as well as metabolism and cognitive ability. It uses cues such as light and darkness to determine when it is time to sleep and, conversely, when the body should wake up.

Biorhythms can change, leading to sleep disturbances. Most people spend most of their lives with a circadian rhythm that ticks along at a steady, healthy pace. However, later in life, this internal clock can begin to lose its accuracy. As a result, older people sleep fewer hours.

With age, the circadian system undergoes significant changes that affect behavioural rhythms, temperature regulation and hormone release, to name a few. While many of these changes may be an inevitable part of development, others may represent the action of pathological processes that simply correlate with age. Moreover, disrupted circadian rhythms are an early warning sign of the development of neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s, as well as cancerous tumours.

Studies show that the sleep cycle in older people shifts, causing them to wake up several hours earlier and go to bed earlier. Similar observations of sleep fragmentation have been made in older rhesus monkeys, hamsters, and Drosophila. Age-related disturbances in human sleep lead to a loss of approximately 30 minutes of sleep every 10 years, starting in the fourth decade of life.

In young people, the body temperature rhythm usually peaks in the early evening and reaches its minimum in the early morning. In older people, this rhythm remains stable, but the temperature does not drop as low as in young people. In addition to the smaller amplitude, a 1-2 hour shift in this rhythm was observed in adults aged 60 to 80 compared to people aged 20 and 30.

What could this be related to? Decreased sensitivity to light may contribute to further changes. There are important differences in the transmission of light through the eyes of young people and the eyes of older people. For example, the human lens yellows and thickens with age, leading to an annual reduction in the total light transmission to the retina of approximately 1% for each year over the age of 18. This yellowing has a selectively stronger effect on shorter wavelengths of light, which convey information about the time of day. The degree of lens yellowing is associated with reports of sleep disturbances in older people.

Hormone release also changes with age. Melatonin and cortisol are of particular interest. For example, melatonin release regulates body temperature and promotes sleep. Total melatonin secretion decreases with age, possibly starting as early as the third decade of life. Data on humans, rhesus macaques, and hamsters indicate that the normal nighttime peak in older people decreases and shifts to earlier in the evening compared to that in younger people. Like melatonin rhythm disruption, cortisol rhythm disruption in older people may indicate progressive neurodegeneration. Seasonal fluctuations in melatonin secretion are also less pronounced in older people.

In mammals, circadian clocks in tissues such as the liver and pancreas regulate metabolic rhythms: glucose homeostasis, lipid metabolism, and detoxification of foreign substances. Many metabolic rhythms show a decline with age. It is believed that the weakening of these rhythms contributes to an increased risk of metabolic diseases in older people, such as diabetes, dyslipidaemia and hypertension.

Fortunately, there are ways to counteract age-related changes. It is important for older people to maintain a consistent sleep schedule, going to bed and waking up at the same time every day. It is also a good idea to go outside and take a walk at the beginning of the day: aerobic activity and exposure to sunlight can help put the brain and body into ‘wake’ mode.