We spend about one-third of our lives asleep, yet most of us know little about sleep and why we need it. Although its function is still not entirely clear, sleep is a universal need for us all and its absence has serious health consequences. Dr Allie Hare, sleep specialist and consultant in respiratory medicine at Royal Brompton Hospital, explores how sleep can affect your health and wellbeing.

What is the circadian clock?

Almost all life on Earth uses an internal biological clock. In organisms as varied as bacteria and humans, physiology and behaviour are fine-tuned to the varied, yet predictable, demands of the day/night cycle.

Creatures effectively ‘know’ the time of day, and these internally generated daily cycles are called ‘circadian rhythms’. A circadian clock, or body clock, also ensures that biological processes in the body occur in the right time sequence.

The circadian clock, is the key factor that defines alertness. Humans have evolved to be awake and active during the day and asleep at night. Whenever this innate schedule for waking and sleep is disrupted, problems can arise.

People who work at night or travel across multiple time zones are required to be alert at a time when the body is physiologically programmed for sleep. During the lowest point in the body’s circadian rhythm, which generally occurs between 2am and 6am, alertness is lower, reaction time is slower, and accuracy is poorer than during daytime hours.

How much sleep do humans need?

The second key factor affecting sleep and wakefulness is our sleep ‘drive’. This is a function of the amount of sleep recently obtained as well as the amount of time since we last slept. Failure to obtain around 7–9 hours of sleep per day rapidly impairs both alertness and performance.

Furthermore, remaining awake for longer than 16 continuous hours significantly impairs performance and safety, especially when the latter hours coincide with the late-night/early-morning period.

Remaining awake on any single occasion for more than 24 continuous hours has significant adverse effects on our concentration, attention, reaction time, memory, and decision-making. This can affect our safety, especially when driving, and our performance at work.

Unfortunately, many of us have lifestyles that don’t align with our natural body clock. Our daily circadian rhythms and sleep/wake cycle allow us to adjust our biology to the demands imposed by the day/night cycle. As time goes on, people are now realising the importance of paying attention to their circadian rhythm and sleep systems.

Why do we need to sleep?

The truth is, we still don’t know very much about why we sleep. Our understanding is growing all the time, but this is still a relatively new field. However, it’s astonishing that, as a society, we know so relatively little about sleep when the average person spends a third of their life asleep. This means that if you live to 90 years old, you will have spent 32 years of your life asleep.

Sleep is a vital part of our lives. It isn’t “lost time” and nor is it just a way to rest when all our important work is done. Sleep is not simply the absence of wakefulness and when you’re asleep, your brain does not shut down. In fact, some areas of the brain are actually more active during sleep than when you’re awake.

Deficits in fundamental aspects of sleep, either in duration, continuity or quality, can have profound health effects that contribute to increased health risks. Extensive research has documented the importance of sleep for our brain.

Sleep is key for learning, for memory and for clearing the brain of waste products. These waste products accumulate in some forms of dementia, and it may be that improving sleep could reduce the risk of dementia later in life.

Can a lack of sleep affect your health?

A number of health outcomes most notably associated with heart disease, including obesity, hypertension and diabetes, are also affected by reduced sleep. Further investigation of 15 studies found that short duration of sleep was associated with a greater risk of developing or dying of coronary heart disease. A long duration of sleep was also associated with a greater risk of heart disease.

Sleep abnormalities are linked to irregularities in blood pressure, fat and glucose metabolism, and weight, and thus may significantly contribute to an increased risk of heart disease. Sleep is likely to affect heart disease because there are day/night patterns of blood pressure, heart rate and insulin sensitivity, all of which can be disrupted when sleep is disturbed.

Sleep also may indirectly influence heart risk via its effects on behaviours such as diet and physical activity. For instance, sleep deprivation disrupts hormones involved in appetite, leading to increased hunger and food intake. Sleep restriction may also lead to fatigue, and result in lower levels of physical activity.

The impact of sleep on mental health

We have seen that deficits in fundamental aspects of sleep, either in duration, continuity or quality, can have profound health effects. However, sleep has another vital role for our health – and regulation of our emotions and mental health – and it’s intrinsically linked to our happiness.

There is a well-documented association between poor or low mood and sleep. We are all aware in our own day to day lives of how our daily emotional experiences influence our nightly sleep and, in turn, how our sleep has an impact on our daily mood.

It was previously assumed that mental health difficulties led to problems sleeping. However, the reverse is also true; poor sleep contributes to the onset, recurrence, and maintenance of mental health difficulties. Various studies have found a two-way relationship between sleep disturbances, anxiety and depression, suggesting that each contribute to the development and are a consequence of one another.

Does improving sleep improve mental health? An analysis of research in the area aimed to synthesise the effect of 72 interventions that improved sleep quality relative to a control condition on subsequent mental health. The findings revealed that improving sleep quality had, on average, a medium-sized effect on mental health, including clear evidence that improving sleep reduced depression, anxiety, and stress.

It was also notable that the authors found a dose–response relationship between improvements in sleep quality and subsequent mental health, such that greater improvements in sleep led to greater improvements in mental health. Taken together, their findings suggest that improving sleep leads to better mental health, therefore providing strong evidence that poor sleep plays a causal role in the experience of mental health difficulties.

Why should we prioritise sleep?

Evidence increasingly suggests that not only is good sleep vital for health but that it improves our mental wellbeing. Sacrificed sleep is often associated with a wide range of negative health impacts and an increased risk of morbidity and all-cause mortality.

Sleep is vital for normal brain function, for memory consolidation and for clearing the brain of waste products, the accumulation of which may be associated with the development of dementia. Sleep is vital for immunity and immune modulation and lack of sleep is associated with increased risks of autoimmune disease, cancer, and cardiovascular disease.

Sleep is vital for cognition, vigilance and decision making. But lack of sleep also impacts general well-being. A night of quality rest seems to help us to feel good and to be able to cope with the emotional challenges of the next day, especially with emotionally painful events.

Sleep also seems restorative in daily functioning, whereas deprivation of sleep makes us particularly more sensitive to emotional and stressful stimuli and events. Sleep disturbances not only restrict our daily happiness, but may even help predict mental well-being, emotional reactivity, and the evolution of mood disorders.

When to seek medical advice

If you regularly struggle to sleep and it affects your daytime activities, causes low mood and irritability, or you have a pre-existing heart condition which you feel could be affected by poor sleep, our consultants can help

Learn more about our specialist sleep disorder clinic, or contact us for further information about our services.