Updated: Apr 27, 2020
Ever tried to make up sleep on a Sunday morning? If you've steadily culled hours from your sleep from Monday to Friday, you'll accumulate a sleep debt: the amount of sleep that you should be getting minus the amount of sleep that you're actually getting.
Let's say that your body requires 8 hours of sleep. That's a fair general rule. If you sleep 6 hours on Tuesday night, you've just accrued a 2 hour sleep debt. The thing is, it's easy to fall into a huge amount of sleep debt because our culture accepts culling sleep and often treats sacrificing sleep as a status symbol, but it's not that simple to pay it off. Imagine if you use your credit card to pay for things that you think you need but you really can't afford. It might seem like it's no big thing at the time, but it adds up. Even if you think that 'you feel fine', your body notices your sleep debt. And your body isn't a friendly community bank that lets things slide. It's a loan shark that has strict deadlines, refuses to negotiate and charges exorbitant interest if you repeatedly miss your nightly instalments. It has clear rules about the instalments required to pay it off - enough quality sleep each night on a consistent basis. No hacks. No short cuts.
It doesn't matter if you're busy, hustling or focused on the grind. You can't escape your biological programming to sleep soundly and properly after each 16 (or so) hours of doing things. And if you actually understand the things that sleep does for you, you'll realise that the last thing you should do is cut it short.
Five fast facts on the physiology of sleep
There’s REM (rapid eye movement) and non-REM sleep.
Non-REM sleep is categorised into four stages – Stages I and II (light sleep) and III and IV (deep sleep).
Your body cycles through the four stages of sleep in the night.
There’s about 3 to 5 cycles in a normal night's sleep (about 90 to 120 minutes a cycle).
Each stage of sleep features different brain waves that characterise our brain activity.
Here's your job. It's simple. Do things to help your body transition in and out of the stages of sleep, and don't do things to fuck it up.
Let's say you sleep from 10 pm to 6 am. If you are sleeping properly and soundly (sleep quality matters just like sleep quantity!), you'll cycle in and out of each stage of sleep.
The different stages of sleep do different things
Your brain does different things in the different stages of sleep. For example:
Stages 1 and 2: light sleep. You start to drift into sleep, you can still be jolted back up.
Stages 3 and 4: deep sleep. This is your restorative sleep. Some of your most refreshing sleep happens in deep sleep. The brain is essentially closed for behind the scenes cognitive maintenance. It does the clean up that it can't do while you're up in the day because your brain is too busy doing other things. It kick starts its sanitation system to drain toxins and flush the brain of metabolic debris. In deep sleep, your brain does its file transfer and memory consolidation. It presses the 'save' button on the things you've learned that day. Your brain organises and processes information in your sleep. 'Sleeping on it' is a real thing!
REM sleep: This is your emotional therapy. REM sleep plays a role in mental health, mood, emotional stress and the ability to control your behaviours. REM sleep resets and recalibrates emotional networks in the brain, particularly the pre frontal cortex (implicated in planning, decision making, and moderating social behaviour) and the amygdala (essential for decoding emotions, handling threats, fear and anxieties).
Is 8 hours of sleep really just 8 hours of sleep?
Look closely at the graph. Do you notice something about the pattern of the stages of sleep? Here's a hint. It's about the amount of deep and REM sleep that you have as you continue to snooze. Each cycle of sleep contains different proportions of deep and REM sleep. It's not like you sleep for 30 minutes in deep sleep, and then 30 minutes in REM sleep, and repeat that all night. As the night goes on, the balance shifts. The first couple of cycles feature longer chunks of deep sleep. Cycles # 3 and # 4 only just dip in and out of stage 3 deep sleep. On the flip side, REM sleep dominates the final cycles of sleep, increasing from about 10 minutes up to about an hour. You spend about 20 per cent of your total time asleep in REM sleep, but the longest periods of REM sleep are in the deeper cycles, after about 6 hours of sleep.
You can't cut and paste chunks of sleep because not all sleep is the same!
So, let's apply this to a common scenario. Say that you go to sleep at 11 pm and get up at 5 am. You'd usually have 8 hours of sleep, but on this instance you only manage 6 hours. You had a busy night, or perhaps you just couldn't fall asleep. You set your alarm, ready to do it all again in 6 short hours. In total, you'll be 2 hours short on sleep - that's a quarter less than you need. You lose 1 or 2 complete cycles of sleep, so your brain doesn't have enough time in the night to do all of the things it's supposed to be doing. It's interrupted after it's only just completed half of the job. That's not ideal. If you sacrifice a cycle (or more) of sleep (particularly before you've completed the first 4 cycles that contain the majority of both your deep and REM sleep), you also disrupt your circadian rhythms. There's another side to this. If you lop off an hour or a couple of hours at the end of sleep, are you just losing 25 % of your sleep? Not really. You're also cutting out a decent chunk of your REM sleep. You’re not just losing 25 % of sleep – you could be losing up to 70 % of your REM sleep. Ie, you’re missing out on most of the REM sleep that you should have had. It's a similar story if you go to sleep late, say 1 am to 7 am instead of 11 pm to 7 am. That short changes your deep sleep. Fragmented sleep is the same thing. It disrupts your body's intricate sequence of the stages of sleep.
Don't be fooled if you feel 'fine'! The first 4 hours of sleep, particularly if that's from 10 pm to 2 am, are the ones that help you to feel less tired as they reduce the desire to sleep that you accumulate during the day (called sleep pressure). That doesn't mean you're good to go! Your body is far from completing the behind the scenes repair and regeneration that happens across a full night's sleep.
Sleep in < 2 hours to pay off some of your sleep debt, but no snooze button!
Sleeping in and napping can help to pay off a sleep debt if you do it right. The trick is to respect your daily circadian rhythms.
Does a day time nap pay off some of my sleep debt?
A nap in the day can help to repay some of your sleep debt. It does that because it releases some of the sleepiness you're feeling that's been building up since the morning. This helps you to function better that day. But more isn't better. The longer you nap, particularly in the afternoon, the more difficult it is to fall asleep later that night because you don't have enough time to build up that sleepiness again.
Nap before 3 pm. If you nap for too long you'll clear a big chunk of sleepiness; that might sound great but it's not all that useful come bed time that night. This issue also applies to sleeping in for too long. In both cases, you might pay off a little of your sleep debt, only to create more debt if you can't fall asleep at the right time later.
Nap for around 20 minutes. If you look at the sleep cycle chart, you can see that if you're falling asleep, you'll drop into a deep stage of sleep in about 20 + minutes. Ever noticed that sometimes you feel even groggier after your nap than before it? That's because you dipped into a deeper sleep before your alarm blared. Or, you could nap for an entire sleep cycle, predict the right time to set an alarm (say, 90 minutes or so) and trial it to see if you can time it just right so that your body rouses easily.
Take home action items
1. Protect your opportunity to sleep each night.
Unless it's a one off thing, or something that happens to be scheduled at night outside of your control. You need enough consecutive hours of sleep because your body relies on completing its sleep cycles one after the other, in the right order. You can't sleep for 3 cycles, lop one off on a busy Monday night and add it back on a Sunday morning.
2. If you're setting an alarm, it should not be set for any less than 8 hours later.
Ideally, 8 + hours and no alarm. If you usually sleep < 7 hours, commit to 8 hours and start banking the extra hours of sleep each night.
3. Have cut off work times and police them.
Don't succumb to the 'I'm on a roll' feeling late at night and keep grinding unless it's really the time and place for that. If you're alert late at night, stop thinking of this as a good thing. Treat it as a symptom of an imbalanced circadian rhythm and hormonal cycle - not a bonus for additional productive time.
Ok, fess up. It is time to change your attitude to prioritise sleep? I've been there. Read my article, I'm slammed and I work really long hours' - How did sacrificing sleep become a status symbol? for four simple tips you can apply to put your sleep first.
4. If you can choose your bed time, sleep at the same times.
Ideal scenario: bed time and up time are the same each day, every day. If you'd like to feel energised and sleepy at the right times, step one is to set up a stable sleep schedule. Repeat daily and let your body find its natural energy pattern. If you don't have a stable daily routine, your body doesn't have one either.
Second best: bed time and up time are within an hour of the same time daily. Eg, bed time is usually at 9.30 pm, but could range from 9 pm to 10 pm. Up time is usually at 5.30 am, but could range from 5.30 am to 6.30 am.
5. Use smart, short naps: before 3 pm and for around 20 minutes.
The purpose of a nap is to help you feel more alert and productive in the day, but not affect your sleep that night.
Dr Satchin Panda, The Circadian Code.
Dr Matthew Walker, Why We Sleep: The New Science of Sleep and Dreams.