Why 90 is the magic number for getting a great night of sleep

Sleep is the foundation of our physical health, mental health, physical performance and cognitive performance.

Since the work of Prof. Matthew Walker and other leading scientists became popular, many more people are getting 7 or 8 hours of sleep each night. However, even when we get that full night’s sleep, we often feel groggy in the morning and tired throughout the day.

As it turns out, how we sleep is more important than how much we sleep. The secret to restorative sleep lies in our so-called "ultradian rhythm": During the day, we go through different levels of alertness over 90 minutes. And at night, we take a rollercoaster ride through various stages of sleep in cycles of 90 minutes.

Here's an overview – or hypnogram, as scientists call it – of how that evolves throughout an ideal night:

The hypnogram shows that 90 really is the magic number for a great night of sleep.

This is due to two major reasons. First, we go through different stages of sleep – light sleep, rapid eye movement (REM) sleep, and deep sleep – in cycles of 90 minutes. When woken up in the middle of one of these cycles, we experience what scientists call "sleep inertia". As a result, we then feel groggy in the morning and even tired throughout the day. The remedy is simple and easy: Set your alarm so that you sleep in multiples of 90 minutes by going for 6, 7.5 or 9 hours – instead of 7 or 8 hours.

The second reason why 90 is the magic number for a great night of sleep is even more important. No matter how many hours we sleep, we need to spend 90 minutes each night in the most rejuvenating sleep stage to set us up for optimal health and performance. That stage is deep sleep. It's when our body repairs and restores its systems, from our muscles to our brain. As you can see from the hypnogram of the ideal night, we can get our 90 minutes of deep sleep mainly within the first half of the night. 

If you often feel groggy in the morning and tired throughout the day despite getting plenty of sleep, chances are your nights aren’t ideal. You probably haven’t spent sufficient time during the first half of the night in deep sleep – but in shallow sleep instead.

Over the following weeks, I'll delve deeper into deep sleep and explain how to get 90 minutes of it each night. Until then, I have a little favor to ask:

Please share this post with one person who might benefit from learning how to get better sleep without sleeping longer.

Until next week,
Christian