Doctors are increasingly waking up to the idea that getting enough sleep is crucial. Last year, the American Heart Association advised doctors that asking patients how many hours they sleep is as important as checking their blood pressure, smoking status, diet, and exercise habits.

But many of us routinely force our bodies to fight sleep. We drink caffeinated beverages by day to stay alert, and alcohol to wind down at night. Some work all night and sleep in daylight. Others switch time zones and struggle with jet lag.

At the same time, neuroscientists are realising that the timing of sleep may be just as important as the amount. People are only just beginning to understand how they might work with, rather than against, the rhythms of their bodies. For example, new research suggests these rhythms may play a role in patients’ responses to cancer treatment.

For millions of years, humans have experienced a consistent pattern of light exposure, with minimal day-to-day variability in the timing of sunset and sunrise. So our genes prepared us to be active in daylight and rest after nightfall.

In 1959, physician Franz Halberg was the first to document humans’ roughly 24-hour self-regulating systems – he called them circadian rhythms, from the Latin words for “about” and “day”.

In 2017, three scientists won a Nobel Prize for showing that humans have a network of timekeeping genes and proteins with daily cycles of rest and activity. These “molecular clocks” in our brain and in every cell modulate our body systems, including hormones, blood pressure, blood glucose, and body temperature.

We are genetically programmed to be hungrier in the evening, which helps us load up on calories before the overnight fast, and less hungry early in the morning, so we can use our energy reserves to go out and hunt for our next meal.

The loop for sleep balances the “sleep drive” with the need to be awake. When we’ve been awake for a long time, our sleep drive kicks in and tells us we need to bed.

Are you an owl or lark?

People whose lives are synchronised with their body clock signals are less fatigued, have better moods, maintain healthier weights, gain more benefit from their medications, think more clearly, and have improved long-term health outcomes, said neuroscientist Russell Foster, who heads the Sleep and Circadian Neuroscience Institute at Oxford University.

Naturally early risers, or “morning larks”, do best when they can wake up early and sleep early. “Night owls” do best if they can sleep until later in the morning.

But when our routines don’t match our biological cycles, the body tries to compensate using responses that evolved to help early humans survive danger, Foster said. It churns out stress chemicals, releases hunger hormones, pumps extra sugar into the blood, and raises blood pressure.

For brief periods, this is not harmful. But when it lasts months or years, people become more vulnerable to cognitive and emotional effects and eventually to cardiovascular diseases, mental illness, diabetes, overweight, and other metabolic disorders, Foster said.

Other studies suggest we also face higher odds for divorce and road accidents.

It’s been estimated that more than half the population in industrialised societies may have circadian rhythms that are out of sync with their schedules. Globally, an estimated one in five workers does regular night shift work, according to a 2020 study by the International Agency for Research on Cancer.

Circadian rhythms shift with age. “Eveningness”, or peak alertness at night, tends to be highest in adolescence. We drift towards “morningness” as we age. One consequence of this is that teachers are generally more alert in the morning, but students are more alert in the afternoon.

A 2016 Rand Corporation report calculated that an employee who works irregular hours, commutes 30-60 minutes each way, and faces unrealistic time pressures at work sleeps on average about 28.5 minutes per day less than an employee who has regular working hours, commutes no more than 15 minutes one way, and is not exposed to unrealistic time pressure and other psychosocial risk factors at work.

This equates to over 173 hours of lost sleep per year.

Ideally, according to Foster and others, society would find ways to mitigate the problems caused by rigid school and work schedules.

What happens during sleep

Researchers have recently discovered that fossilised plants living more than 250 million years ago – just as dinosaurs were starting to appear – already had circadian rhythms that drove them to fold their leaves at night and reopen them during the day.

All animals sleep, but it can look widely different depending on the species. According to a Boston University database of the sleep characteristics of 133 mammalian species, red kangaroos sleep for just under two hours in a 24-hour period, while armadillos and bats can sleep for nearly 20 hours.

Some animals, including dolphins and whales, sleep unihemispherically: one half of the brain sleeps at a time, to allow them to be constantly alert.

Sleep itself has cycles, in which the brain and body move through phases, marked by varying brain activity. In the deepest phases of sleep, the brainwaves are slowest. The lighter phases have more rapid bursts of activity.

The most intense dreams usually happen during Rapid Eye Movement (REM) sleep, when brain activity, breathing, heart rate, and blood pressure all increase, the eyes move rapidly, and muscles are limp. Scientists believe dreams in REM and non-REM sleep have different content – the more vivid or bizarre dreams usually happen during REM stages.

During sleep, the brain is extremely active. In a typical night, different regions become engaged as we move through the four main stages of sleep.

If conditions are right, nerve cells in the brain and a cluster of cells in the hypothalamus, the brain’s control centre, signal that it’s time to transition to the first stage of sleep. Later, parts of the brainstem will relax our limbs.

Brainwaves slow significantly. Eyes move behind closed lids, but muscle activity is minimal. Signals between the nerve cells decline, allowing the brain to process only important information.

During this time, some kinds of memories are consolidated. Memory traces are transferred from a temporary store in the hippocampus to the cortex.

Waste from brain cells’ daytime activity is flushed out in cerebrospinal fluid through a newly identified pathway called the glymphatic system. A region in the upper brainstem kickstarts the move into REM sleep. This phase plays an important role in dreaming, emotion processing, and brain development.

Brain activity increases, the eyes move rapidly, the heart rate speeds up, and breathing becomes irregular.

During REM sleep, the cortex – responsible for cognition and emotion-processing – is activated in some regions and deactivated in others. Voluntary muscles in the body are paralysed, so we do not act out our dreams.

After cycling through non-REM and REM sleep around four to five times, the basal forebrain and other structures receive signals to start exiting sleep.

Finally, light exposure to the optic nerves triggers the release of cortisol and other circadian cues that help start the process of waking up.

Don’t miss part two of this article, which discusses sleep disorders, the costs of insufficient sleep, and how to get better rest.