What happens when we sleep?

When we drift into sleep, our brain kicks into gear performing all the repair and restoration tasks it cannot do in our waking hours.

Although it doesn’t look like much is happening on the outside, inside it’s a hive of activity.

SLEEP STAGES

Sleep, as opposed to rest, is defined by a loss of awareness of our surroundings and, as opposed to a coma, it is a rapidly reversible state, meaning we can be easily woken. When the sun goes down, assuming we don’t turn on all the lights in our house, a cluster of cells inside our brain called the suprachiasmatic nucleus responds by signalling to the pineal gland to produce melatonin, which makes us feel sleepy.

When we fall asleep, our brain takes a ride that occurs in four stages: non-rapid eye movement (NREM) 1, 2 and 3, and REM. Each stage of NREM sleep becomes progressively deeper and accounts for about 75 to 80 per cent of our total time spent asleep.

Stage 3 NREM is what we typically refer to as ‘‘deep sleep’’ as our brain waves slow into Delta, and we become hard to rouse. REM sleep, the final stage, is associated with vivid dreaming but is not considered restful.

An average night’s sleep consists of four to five sleep cycles, which take about 90 minutes each. NREM generally dominates the first cycles, but as the night progresses, the balance flips and REM sleep dominates each cycle.

DEEP CLEAN

As we slide deeper into sleep, our body temperature and heart rate drop, brain activity decreases and electrical patterns change, as thousands of brain cells start singing in synchronicity. Though the first phase of sleep – NREM – is largely relatively light sleep, it serves an important purpose. During NREM, our body consolidates memories, grows and repairs itself, bolsters immune function and restores energy.

During sleep, our brain also performs a ‘‘deep clean’’ on itself, clearing away toxic waste products. This deep clean or brain wash, researchers believe, may be critical in protecting against dementia and general cognitive decline.

Ron Grunstein, the head of the Sleep and Circadian Research Group at the Woolcock Institute of Medical Research, says the discovery that sleep ‘‘detoxifies’’ the brain was made in 2013.

‘‘There is a price to pay for being awake: your brain builds up all sorts of crap – toxins and things,’’ he explains. ‘‘That’s part of the process of your brain being active.’’

To clear that out, the glymphatic system (a network of vessels in the brain that acts as a sewage system) activates during sleep.

‘‘One of the purposes of sleep is to reduce the cost of being awake – you counter the cost. If someone stayed awake and never slept, they’d fry up the brain, basically,’’ says Grunstein.

ABSURD ACTIVITY, CREATIVE SOLUTION

As our brain cycles into REM sleep for between 10 and 60 minutes (cycles increase in length as the night goes on), our breathing becomes fast and irregular, our eyes move rapidly and our brain waves are chaotic (mimicking our awake state). A part of our brain called the thalamus activates, sending the cortex images, sounds and other sensations that form our dreams. But during REM our bodies become temporarily paralysed to stop us from playing out our dreams. It’s also when our testosterone levels peak and our brain works to strengthen memory associations. This preservation of memory is not just about where we left the car keys but retaining skills and information. After a good night’s sleep we are likely to perform significantly better on the sports field. Sleeping well for a week before exams can improve performance by as much as 25 per cent. Even the absurd activity that is dreaming – which occurs mostly during REM sleep – serves an important function: it boosts creativity and problem-solving by allowing our brains to make wild connections we wouldn’t normally make. University of Sydney sleep medicine researcher and clinician Delwyn Bartlett says we need every stage of sleep. ‘‘There isn’t one stage that is more important,’’ she says, adding that each stage activates different parts of the brain while slowing down another, a harmony that optimises our physical and mental health in a symphony of ways.

SLEEP TAKE HOME

Every sleep stage is equally important and allows our brain to get to work repairing, deep-cleaning, retaining and restoring creativity, imagination and memory.

NEXT TUESDAY

We’ll investigate what a perfect night’s sleep looks like. (Hint: it’s dependent on many factors – including your chronotype.)