Between dozing off and the alarm shrieking, our bodies and brains undergo quite a few changes. Sleep researchers call the first stage of sleep N1. It is light, and if woken from it we might not realise we had drifted off in the first place. Our night-time voyage is well established by the time we enter the second stage of sleep, N2. This can be identified in the laboratory by distinctive brainwaves such as sleep spindles, involving a sudden burst of brain activity. N3 is referred to as deep sleep or slow-wave sleep because of the brain activity, which is more comparable to rolling waves. Eventually, we reach rapid eye movement (REM) sleep – a stage characterised by brain activity not unlike that seen during our waking lives. Vivid dreams and bodily paralysis also occur – perhaps the latter keeps us safe, by preventing us from acting out the former. Adult sleep cycles take about 90 minutes.
Two main processes control when we sleep. The first is called sleep homeostasis and refers to our sleep drive. The longer we have been awake, the greater our pressure to fall asleep. This alone can’t explain why we might struggle to sleep during the day, despite having been up all night; or find ourselves nodding off in the evening even though we snuck a daytime nap. Instead, the circadian process helps to explain such occurrences. This clock-like mechanism is controlled by a central pacemaker in the brain. Our internal body clocks often run at slightly over 24 hours a day, so we use zeitgebers (time-givers) from the environment, to tweak our internal clocks so as to fit with the world outside. Cues come from all around – including the time we eat or exercise – but none is as important as light exposure. Light helps our brain know it is time to be awake and darkness allows us to produce melatonin, giving our bodies a sign that it is time to sleep. Without cues from the environment, bedtime would typically become later night-on-night, and soon we’d find ourselves ready to sleep when we should be waking up.
Recommended sleeping hours
Our sleep changes throughout our lives. At birth, babies do not move through sleep stages as they will as adults. Instead, they start their night in active sleep (much like REM) and progress into a stage known as quiet sleep. A premature baby may complete a sleep cycle in as little as half the time it takes an adult. The circadian and homeostatic processes develop gradually; by six months, the body clock is well-established and synchronised to the surrounding world. Babies’ sleep can be difficult for parents to cope with, but a plethora of advice is available, such as that on the website set up by the Pediatric Sleep Council.
As babies develop into pre-schoolers and then start school, the amount of sleep they need decreases. The composition of sleep changes, too. Whereas newborn babies spend about 50% of their sleep in a REM-like state, this reduces to 25% by the age of two. REM sleep is most abundant at the stage of life during which the brain is most plastic – and when connections are being formed most rapidly. This hints at the importance of REM for the developing brain, and one of its functions may be to support the developing visual system.
Over time, the challenge of a baby waking around the clock can be replaced by other night-time issues, such as bedtime resistance of a toddler. Establishing routines early in life can be helpful, and research has found that the more often a child has a bedtime routine, the better they sleep. There are endless other tips to support sleep, including avoiding light at night: exposure can suppress melatonin. Televisions should also be kept out of the bedroom. Pre-schoolers with TVs in their bedrooms have been reported to have poorer sleep.
As well as length, sleep timing changes throughout our lives. The most dramatic shift occurs during the teenage years. Sleep can easily shift two hours later in adolescence, compared with that of younger children or older adults. This change is found in teenagers around the world and in other mammals too, and has been linked to puberty. For example, puberty has been associated with a delay in the start and end of daily activity in marmoset monkeys. The processes leading to the shift in sleep timing that occurs during adolescence have been referred to as the “perfect storm”, involving changes in the homeostatic and circadian mechanisms in concert with social pressures and societal demands.
The mechanisms underlying this shift are complex. For example, some data suggest that adolescents have a lower sleep drive than children during certain parts of the evening, making it harder for them to nod off. With increased development there does not seem to be a greater sensitivity to evening light, as was once considered likely, but rather there might be greater exposure to light that could delay the body clock. This light could come from multiple sources including smartphones, tablets and TVs. Increased autonomy granted by parents and greater social obligations could also result in a later bedtime.
Although it is clear that sleep is important for development, many children and adolescents may not be getting enough. For example, a report by the US Centers for Disease Control and Prevention, published last year, noted that more than half of the middle-school children assessed reported getting less than the recommended amount. This increased to nearly 75% of those in secondary education.
Sleep can easily shift two hours later in adolescence, a process that has been referred to as the ‘perfect storm’
This widespread issue needs to be addressed. Policy decisions, such as pushing back school start times for adolescents to align with their circadian patterns, can result in more sleep. Whether a bedtime is set by parents (and its time at which it is set) is also linked to the amount of sleep children get. The value of educating children about sleep might seem intuitive, but research has suggested that education alone does not always result in the desired behaviour change. Those who miss out on sleep during the week, might want to catch up at the weekend – and this can be helpful. However, we shouldn’t deceive ourselves that this is as good as a consistent sleep routine. Substantial changes to our sleep timing can result in a jetlag-like state (social jetlag). A shift in sleep timing at the weekend can mean that on Sunday evening we need to traverse multiple time zones in order to cope with an early start on Monday morning. Going to bed earlier than we did on the preceding nights can mean that it is difficult to nod off and we end up missing out on sleep. Social jetlag has also been associated with various difficulties including alcohol consumption, depression, metabolic dysfunction, and obesity.
At about 20 years of age, there is a return to earlier sleep timing, and this shift has even been considered to provide biological evidence of the end of adolescence. Sleep-length recommendations in early and mid-adulthood average about eight hours a night. However, during these rush-hour years, many people report that it can be difficult to achieve. At least some recent data suggests that things could be improving, and that sleep duration is increasing. Sleep remains important as we hit our senior years, yet it can be more difficult to obtain than ever before. Reasons are plentiful and could include a reduction of cells in brain regions important in bringing about sleep, and changes to our eyes which filter light in a different way. Health difficulties and social reasons, such as lifestyle changes linked to retirement, can also play a part.
Women and sleep
While relatively under-researched, different aspects of sleep have been linked to gender. The adolescent shift in sleep timing differs in males and females, with the shift peaking earlier in females compared with males, perhaps reflecting their earlier physical development. After puberty, males typically have later sleep timing than females, a trend that disappears once menopause is reached.
Puberty also rings changes for those horrible sleepless nights that we all experience. Whereas insomnia is reported with similar frequency in young boys and girls, after puberty, it is more common in females, remaining that way into older adulthood. Daniel Buysse, professor of sleep medicine at the University of Pittsburgh, notes: “Men and women also have differences in sleep physiology, specifically greater amounts of deep sleep in women. Paradoxically, one might predict better sleep in women as a result.”
Depression, commonly associated with insomnia, becomes more pronounced in females than males during adolescence
So, why might these gender differences appear? Links to other conditions could play a part. Instances of depression, which are commonly associated with insomnia, become more pronounced in females than males during adolescence. Could the increased rates of depression in females after adolescence to some extent also help to explain the rise in insomnia?
Another possibility is that there are gender differences in the way that men and women report symptoms. Prof Derk-Jan Dijk, the director of the Surrey Sleep Research Centre, thinks this may be the case. He tells me: “Our data show that in healthy people without sleep complaints, the association between subjective sleep quality [self-report] and polysomnography [an objective measure of sleep] is much stronger in women than in men.” This might suggest that women are more accurate than men at reporting certain aspects of their sleep.
Buysse also notes that “differences in stress sensitivity, particularly interpersonal stress sensitivity, may also play a role. Women are generally more attuned to social evaluation and social stress, which can disrupt sleep.”
Hormones are likely to be important when it comes to sleep. This chimes well with a study revealing a reduction of sleep efficiency (or the proportion of time in bed spent asleep) as a woman moves through her cycle, dipping to its lowest point in the premenstrual period. This may be a function of reproductive hormones declining during this phase – during which some also experience premenstrual symptoms. Subsequently, menstrual cramps can also disrupt sleep.
Changes associated with pregnancy also bring novel night-time challenges. Women commonly report insufficient and poor quality sleep as well as daytime sleepiness. Disorders such as restless legs syndrome and sleep-disordered breathing may also develop during pregnancy. Jodi Mindell, a professor of psychology at Saint Joseph’s University in the USA, notes that “throughout pregnancy over 95% of women report waking at least once per night. Sleep disturbances are a universal issue for pregnant woman and have even been found to be associated with premature labour and increased likelihood of a caesarean birth.”
When babies arrive there is no let-up, and disrupted nights can continue for months, and even years. In a study of 29,287 parents, it was found that even by two to three years of age, children wake up around once a night. “Sleep disturbance is an expected part of becoming a new mother,” says Katherine Sharkey of the Alpert medical school, Brown University, “but there are no guidelines for perinatal women and the clinicians who care for them regarding how severe and for how long shortened and disturbed sleep should be tolerated before treatment is considered during pregnancy and the postpartum period.”
I would wake up feeling intense heat around my face and neck. I felt trapped in the burning furnace of my body
The arrival of the menopause brings more sleep challenges. Those who are post-menopausal more frequently report short sleep, have trouble sleeping, and wake up feeling unrested as compared with their premenopausal counterparts. In my book, Nodding Off, I quote a lady in her 70s describing her sleep during the menopause: “My joints felt more painful and particularly so at night. I would wake up feeling intense heat around my face and neck. I felt trapped in the burning furnace of my body … Even in the coolest of nights I threw off all my covers and still felt internally hot, though aware that my feet were freezing …”
As described by a document from the US Society for Women’s Health Research, sleep problems during the menopause may be caused by a plethora of issues, including increasing rates of sleep apnoea, hot flushes and night sweats, and symptoms of depression and stress that may occur at this stage of life. Treatments include those that are also recommended at other stages of life, such as cognitive behavioural therapy for insomnia and addressing associated sleep problems such as apnea, as well as others that are more specific to this stage of life, including hormone therapy.
Women differ, so how are group comparisons helpful? Understanding trends can be useful. If we know that insomnia is a particular issue for older females, “well-women” checks might want to ensure that this issue is always discussed. If different factors predict poor sleep in women and men, this could be helpful for developing preventions for disturbed nights for both. Comparing men and women has also revealed gender differences in the metabolism of certain hypnotic sleep medicines – with the implication that when these are prescribed, males and females should be given different quantities to avoid potentially dangerous side-effects. More work on gender differences will result in better nights for all.