#146 - How Sleep Changes Across the Human Lifespan

Matt Walker (0:02)

Hi there, it's Matt here, and welcome back to the podcast. For this episode, I want to travel a single question across the whole of a human life.

How does sleep change as we age, and why do certain moments make it so much harder? It is the question a parent asks while holding a baby who is wide awake at 3 in the morning. The question a teenager asks when sleep will not come, and the alarm is set for 6

It is asked during pregnancy through the heat and disruption of menopause after a bereavement and at the end of a working life. There is no single answer, and I want to say that at the outset. Human lives do not unfold as seven identical steps. Not everyone becomes a parent, not everyone experiences pregnancy or menopause.

Grief and trauma can arrive at any age, and retirement can be chosen, imposed, delayed, or never reached at all. So the stages I'm about to describe are not deadlines, diagnoses, or destinies. They are patterns seen across groups of people. A population average can tell us what is common. It cannot tell any one person what will happen to them. With that care in place, let us begin where sleep itself begins.

A newborn's sleep is not a smaller version of adult sleep. It is organized differently from the start. Newborns spend a much larger proportion of the night in what researchers call active sleep, the early life counterpart of rapid eye movement or REM sleep. In an adult, REM usually occupies around a fifth to a quarter of the night. In a newborn, active sleep can occupy roughly half.

That difference is thought to support the extraordinary development taking place in the infant brain. Though one qualification matters, we cannot know whether a newborn's inner experience resembles the dreams an adult can remember and describe. Active sleep is visible and measurable. The content of an infant's mind is not. The timing differs too. Adult sleep cycles average roughly 90 minutes. Infant cycles are shorter, often around 50, and sleep is scattered across the day and night, while the circadian system is still assembling itself over the first two or three months. This is why early sleep can feel so relentless. But waking is not the same as crying, and a baby who stops signaling has not necessarily stopped waking. Consolidation happens, but not in a straight line. In one large study, only about one in six three month olds met the definition of sleeping through the night, and waking remains common well past six months. And there is one boundary that no sleep advice may cross. In the United States, the American Academy of Pediatrics recommends placing an infant on their back on a firm, flat, separate sleep surface with no pillows, loose bedding, or soft objects in the caregiver's room for at least the first six months.

A sofa or armchair is an especially dangerous place to fall asleep with a baby. Families elsewhere should follow their own public health guidance. Frequent waking is not evidence that you have failed, that your baby is defective, or that you missed some narrow window in which sleep had to be fixed. The next great shift arrives with puberty, and it is not a matter of attitude or discipline. As puberty progresses, the circadian clock tends to run later, and sleep pressure, the drive to sleep that accumulates across the waking day, appears to build differently, allowing many adolescents to stay awake far later than they could as children. Evening light delays the clock at any age, and phones add stimulation and displace bedtime. But a teenager's late sleep is not adequately explained by a glowing screen or a shortage of willpower. The collision comes when a later biological night meets an early social morning. Teenagers aged 13 to 18 are generally advised to obtain 8 to 10 hours in each 24, and an adolescent who cannot fall asleep until late, but must rise at 6 or 7 may be placed in a schedule that makes that range nearly unreachable. Sleeping much later at the weekend provides some genuine recovery, though a large swing in timing can make Sunday night harder still.

Later, school start times have repeatedly been associated with longer sleep and less daytime sleepiness, with the size of the benefit varying between schools and studies.

Morning daylight, a reasonably stable wake time, and dimmer light before bed may help, but they work best offered as support rather than surveillance.

If you live with a teenager who seems unable to wake, begin with curiosity rather than contempt. Ask when sleepiness actually arrives, what the schedule demands, and whether anxiety, depression, medication, substance use, or another sleep disorder may be involved. Severe daytime sleepiness deserves attention, not ridicule.