Wednesday
Dec282011

Sleep Debt 

Imagine that it's midnight: you are sound asleep and the alarm is set for 5:30am as usual.

A noise wakes you up! You sit bolt-upright in bed trying to work out what it was. Eventually you get up to have a look and can't find anything so you go back to bed.

You are no longer sleepy, in fact you are now lying awake looking at the numbers on the clock as they get ever closer to the time that the alarm is set for. You stare at the clock for a whole hour...

...then eventually drop off to sleep.

The alarm goes off and you get up feeling a little more groggy than usual.

What you didn't realise is that while you were staring at the clock, you were racking up debt - sleep debt.

You don’t need to try very hard to build up a sleep debt.

Imagine losing just an hour of sleep a night for a week, the odd late night here, early morning there etc – that would take nearly a whole night of its own to pay off, not to mention the 8 hours that you’d normally spend asleep on that night.

You've been deprived of seven hours of sleep, and your brain is not going to let you forget it! The debt is going to come looking for you when you least expect it. You might be watching the TV, or sitting at your desk, or worse - driving when it creeps up on you and demands that you pay it back!

…and pay it back you will. It has a long memory as far as sleep debt is concerned... or so the current thinking says.

In his book, “The Promise of Sleep”, Dr William Dement (the sleep pioneer who named the stages of sleep and conducted many groundbreaking experiments into sleep and sleep deprivation), states that as far as his experiments were concerned, the sleep debt was still valid for at least two weeks after it was racked up. That isn’t to say that after two weeks, the debt is forgotten, but just that experiments had only explored sleep-debt for two weeks.

But what does that actually mean in practice? Does it mean that you need an additional seven hours of sleep to rebalance things? To be honest, there is division about how much of the debt needs paying back, and it may well vary from person to person.

I decided to look at my own data to see if I could find any clues.

This is a night of my sleep while I was in hospital recently with my son. I had to wake up every two hours to perform a procedure. 

There are solid chunks of wakefulness throughout the night, along with a significant drop in REM.

I normally get around 1:40h of REM and 40m of deep sleep. It's interesting that although my REM suffered, my deep sleep didn't. This suggests that the brain values deep (slow wave) sleep more than REM, so it makes it a priority before it catches up on REM.

Then the following night we were able to have a lie in on the ward, during which we caught up and paid back some of the debt (resulting in more REM and a higher score). 

The bulk of the debt was paid back on the first night, although the debt continued to be paid back the following night too. 

This is easier to see with the following graph covering the two consecutive nights of sleep debt (the first of minor debt and the next of more significant debt shown above) along with the two nights of payback. 

Besides charting your sleep every night, there are other ways of measuring sleep debt. 

People get used to being tired in the day and consider it to be normal, so simply asking someone how tired they are is a little too subjective. The Epworth Sleepiness Scale was designed to be more objective.

You can take the short test here http://www.predictonline.com/epworth.htm

Most of us are familiar with the fact that if you are tired then you can pretty much drop off to sleep as soon as your head hits the pillow. The technical measure of the time it takes to get to sleep is called "Sleep Latency".

Sleep latency is actually a good marker of sleep debt - the quicker it takes for you to get to sleep, the greater your debt. Eventually the time to sleep onset reaches zero, then you have what is called a "sleep attack" - when the brain's desire for sleep is so great that it just blacks you out, even if just for a few moments. This can have disasterous effects, especially if you are driving at the time!

The brain is choosy about the currency that you use to pay back the debt. If you missed out on REM then it wants REM paid back (this is called REM-rebound). If you missed out on slow-wave sleep (deep) then it wants paying back in slow-wave sleep. 

This screenshot of the week leading up to (and including) the sleep debt shows how an unusually low amount of REM is caught up on:

I imagine that the brain would arrange the payback at the expense of the light sleep (N1 and N2) which largely appears to me to be "padding" to keep us asleep and out of trouble (evolutionary hang-up) or a vehicle necessary for the two types of restorative sleep: REM and SWS. I tend to think of this as a "more concentrated" form of sleep.

This is nicely shown by this graph of my REM for the above period, shown as a percentage of total sleep:

When the need arises, REM can form a larger part of total sleep, proving that it isn't accrued in relation to time spent asleep, that is to say that there isn't a formula that says "For every hour of sleep, 15 minutes of it will be REM".

So it seems unlikely that a total-sleep payback of an hour-for-an-hour of whole sleep is likely, although this may be true for REM and slow-wave sleep.

This all seems very well for small debts but it didn't seem to work for a larger debt...

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