WHAT HAPPENS TO SLEEP RHYTHMS AS WE AGE

AS WE pass through life, our sleep changes both in quantity and quality.

A newborn will sleep for two-thirds of the day, but, by adulthood, we tend to sleep between six-and-a-half and eight-and-a-half hours a night.

Nor is sleep a static state — it has multiple stages. As we drift off, we enter stage 1 sleep, also known as drowsiness; then stage 2, light sleep, when brain activity slows further.

Stage 3 is deep sleep: within 30 minutes or so of drifting off, brainwaves slow considerably, but increase in size. The final stage, which we enter after around 60 to 75 minutes, is rapid eye movement (REM) sleep, so-called as our eyes rapidly dart back and forth.

During REM sleep, the brainwaves look to be highly active — a little like being awake — and it is then that we most obviously dream.

As adults, we move through these various stages usually four or five times a night, with the majority of deep sleep in the first half of the night and the majority of REM sleep in the second. As newborns, we spend around half of our slumber in REM sleep, while in adults it’s 15 to 25 per cent, gradually falling as we approach old age.

The proportion of deep sleep changes, too, being roughly 15 to 25 per cent in adulthood, but dropping a little in the elderly, usually replaced by Stage 1 and 2 sleep. As we get older, the number of brief awakenings throughout the night also increases.

Deep sleep is thought to be when our brains do more of their housekeeping in clearing out waste substances and toxins.