2014 article

Source witness.theguardian.com

We are all subject to a sleep cycle of about 90 minutes duration. In that period, we go from light sleep to deep sleep and back when we are asleep; or we go from feeling tired to being alert and becoming tired again, when we are awake.

Use your sleep cycle by noting when you wake up in the morning, without the use of an alarm clock. You’ll notice that if you wake up unaided by an alarm at 06.00 and then go to sleep, the next time you’ll wake up, is around 07.30. If you also woke up earlier, for instance from a dream, that would be around 04.30.

These are your awake-times in the morning. But most people calculate the time they need to get to work, and count backwards to the time they set their alarm – often ensuring that they wake up between their awake-times, when their bodies are in deep sleep. Instead of waking up fresh, you stagger out of bed like a zombie.

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The entire article can be read at this link.