The ‘Sleep Calculator’ is just unscientific hype

Sleep Health Foundation says that the assumptions that the Sleep Calculator makes, and the advice it gives, are not based on scientific evidence.

calendar icon
September 12, 2023
time, clock icon
min read

There have been a number of media stories recently on the supposed benefits of calculating when you should go to bed so you wake more refreshed when the alarm goes off in the morning. It gives a series of time recommendations like 9.45pm or 11.15pm if you want to wake up at 7am. The idea is to align your bedtime to your 90 minute sleep cycles, so you wake after a complete cycle in REM sleep (when we have structured dreams) and thus feel more refreshed.

However, the Sleep Health Foundation says that the assumptions that the Sleep Calculator makes, and the advice it gives, are not based on scientific evidence.  The Calculator takes two scientific facts, firstly that we have sleep cycles across the night of approximately 90 minutes in duration and secondly that waking from REM sleep is more likely to make you feel refreshed, and massively over-generalises.  Following the advice may make you awake refreshed in the morning – but it may not. Here is why.

  1. Sleep cycles across the night are only approximately 90 minutes in length. There are lots of individual differences in cycle length and the variation may be from around 60 to 110 minutes.  There may also be unpredictable differences in the same individual from night to night.
  2. It can be hard to predict how long it takes to go to sleep. The same person may take quite different times to fall asleep on different nights and different people take different lengths of time.  If you keep routine sleep/wake times and go to bed when your body clock is expecting it, you are likely to fall asleep faster than if you go to bed, say, 2 hours before your body clock is on its ‘Let’s go to sleep now’ setting.
  3. Waking up during the night is normal but time spent awake is unpredictable. We know that waking up during the night is perfectly normal and occurs quite frequently in almost everyone (although they may not be aware of it).  Some people can go straight back to sleep while others will visit the bathroom, make a warm drink, start to worry, have a cigarette and/or watch some TV when they wake up.  While we don’t recommend the last three of these for helping get a good night of sleep we know they happen.

We know that people often feel most refreshed when they wake from REM sleep after having a good night of sleep.  Parts of the brain are more active in REM sleep than when awake. We get more REM sleep in the morning, especially if we are not sleep deprived.  If you go to bed sleep deprived, or sleep when your body clock is not expecting it, you may have more deep sleep (rather than REM sleep) in the morning and therefore wake up to your alarm feeling groggier.

So the best advice for waking refreshed in the morning is to follow Good Sleep Habits.

Pamper your body clock with routine sleep/wake times.  Don’t for example, get too little sleep during the working week and then sleep in for hours on a weekend – you will just wake up to your alarm on Monday morning feeling like you need more sleep.

The Hilary's Blinds Sleep Calculator

Emeritus Professor Dorothy Bruck
Sleep Health Foundation