👀 Here’s how we figured out there are 2 types of sleep

Armond Aserinksy found himself in an unusual position. He sat patiently on the old army cot as his father scrubbed his scalp and the skin around his eyes with acetone, taped electrodes to the his head and plugged the leads into a switch box over the bed.

Recently, Eugene Aserinksy had been working late. He had begrudgingly accepted a position as a grad student in the lab of an eccentric sleep scientist by the name of Nathaniel Kleitman (yep, the same guy who shut himself in a cave for a month). His wife was unwell and money was tight; he’d been stealing potatoes just to get by. Kleitman could offer financial support in the form of a small loan… and the suggestion that he should eat chicken necks to get by.

Now, on a cold December evening in 1951, he found himself working out of converted army barracks - taping electrodes onto his 8-year-old son, Armond, under the light of a kerosene lamp. It wasn’t long before Armond fell asleep. But Eugene forced himself to keep awake in the next room, sitting in front of the old brain-wave machine he had dragged there. As the hours passed, he noticed that the pens tracking his son’s eye movements - as well as the pens registering brain activity - were swinging back and forth, as if Armond had been startled awake and was looking around in shock. Aserinsky went in to check on his son. But Armond’s eyes were glued shut; the boy was fast asleep. Something mysterious was taking place… Aserinsky knew he’d found what he was looking for.

About a year later, together with his advisor Kleitman, he published a paper that would change our understanding of sleep forever:

REM sleep is often associated with vivid dreaming. It is characterised by a set of distinct physiological markers: brain activity similar to when you are awake, rapid eye movements and a state of muscle paralysis that completely immobilises the body. (Aserinsky at first thought of calling the phenomena “jerky eye movements,” but decided against it.) REM sleep remains just as mysterious as the day we discovered it… today scientists are still unravelling its secrets.

Thanks to Aserinsky, we now separate sleep out into two distinct stages: non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Here’s a quick summary of the differences between the two:

Sources: NHLBI (2003), Somers et al. (1993), Madsen et al. (1991b).

😶 An exposé: the story of my sleep cycles last night

Following Aserinsky’s breakthrough, it didn’t take long for scientists to realise that NREM and REM sleep repeat in cycles of approximately 90 minutes throughout the night, with NREM mostly occupying the first half of the night and REM beginning to feature more in the second half.

NREM itself can also be further broken down into four sub-stages. With stages 1 and 2 being “light sleep” and stages 3 and 4 representing the deepest levels of sleep (“deep sleep”).

An individual sleep cycle lasts approximately 90 minutes, during which the brain cycles from slow-wave sleep to REM sleep. But your sleep cycles change throughout the night. For example, if you look carefully at my sleep graph you might notice that the first sleep cycle looks a little shorter than the ones the follow. It turns out that the average length of the first NREM-REM sleep cycle is 70 to 100 minutes. The second and subsequent cycles tend to last slightly longer (e.g. 90 to 120 minutes).

The changing NREM-REM balance as you pass through sleep sleep cycles may have finer consequences for brain function. For example studies show NREM sleep affects how the brain prunes its neural pathways, while REM sleep is involved in building memories and emotional processing. Studies show that disrupting this natural sleep profile can have consequences for your cognitive function and memory consolidation.

Unfortunately, on this particular night I didn’t get much REM sleep. In adults, REM sleep increases as the night progresses, with more of it occurring closer to your wake-time in the morning. I could speculate that this has something to do with my late bedtime pushing REM towards the morning - followed by a rude awakening just as I was finally beginning to get into REM!

🔋 The #1 way to maximise sleep efficiency and wake up fully rested

Sleep deprivation, whether of REM or NREM sleep, can have detrimental effects on physical and mental health. Studies have linked inadequate sleep to increased risk of various health conditions, including obesity, cardiovascular disease, and mood disorders.

On an average night, adults typically need to complete at least four or five sleep cycles per night, or 7 to 9 total hours of sleep. However, there is a large degree of variability from person to person and from night to night. I find it usually takes 5 cycles for me to feel fully refreshed!

The main thing to be aware of is that waking up in the middle of a sleep cycle can leave you feeling tired or groggy as you start your day. The best way to prevent this happening is getting enough sleep on a regular basis. By giving your body the time it needs to complete the 4-5 cycles you need, you will find that you are able to “wake up naturally” - and after a while this can become a habit without even needing an alarm clock!

To wake up feeling your best, therefore, it’s less about a certain magical bedtime and more about being consistent.

For example, this 2009 study of Taiwanese university students found that irregularity of bedtimes was significantly associated with a decrease in average sleep time per day. On top of this, a frequently irregular bedtime had poor sleep quality, even after adjusting for the sleep time.

🔍️ Actionable insight: smart alarms (sleep efficiency hack for irregular bedtimes)

What happens when things go wrong and you don’t get into bed on time? A general rule I use for setting an alarm is a multiple of 90 minutes (1.5 hours) plus an extra 30mins for transitioning into/out of, sleep. This helps me avoid waking up in the middle of a sleep cycle. It’s also why I find that generally feel better off with 6.5 hours in bed, rather than 7!

Note that this is a short-term measure (nothing beats a full night of sleep). It’s also based on the individual, some people might find that a multiple of 1.5 hours plus a whole extra hour works much better for them!

🛠️ My current Sleepstack:

• Screen filter software: Tools like f.lux (desktop), Twilight (Android) and Night Shift (iOS) change the colour of your display to based on the time of day to reduce overstimulating wavelengths.

• Sleep mask: I use this one at the moment. Light affects melatonin secretion even when your eyes are shut. A sleep mask is also a great alternative to investing in a set of blackout curtains; I never sleep without one now.