How Much of Each Sleep Cycle Should I Get Each Night?

A healthy night is roughly 5% N1 (the bit where you are asleep but would testify otherwise), 50% N2 (actual proper sleep), 20% N3 (deep sleep, where your body runs maintenance with the urgency of a pit crew), and 25% REM (where your brain processes emotions and tests hypotheses like "what if your old headmaster was actually a fish"). You do not need to manage this distribution. Your brain handles it automatically, the way your heart handles beating. Your only job is to stay in bed long enough for the whole programme to run. This is apparently asking too much of most people.

The trouble begins when you start shortchanging the schedule.

What Happens When Deep Sleep Gets Cut?

Deep sleep clusters in the first two or three cycles of the night. This is the stage where growth hormone is released, the immune system receives its orders, and your muscles are repaired with a thoroughness that suggests your body does not trust you to look after them during the day, which, given your posture, is fair.

Go to bed at 1am instead of 10:30pm and you do not merely lose two and a half hours. You lose the two and a half hours that contained the highest concentration of deep sleep, which is like missing the first act of a play and then wondering why nothing in the second act makes any sense. Alcohol suppresses deep sleep further. So does caffeine. So does a warm bedroom. If your evening routine involves all three, your deep sleep is not being short-changed. It is being mugged.

The result is waking up after what should have been a perfectly adequate number of hours and feeling as though you spent the night in a cement mixer. Your body needed repairs. It did not get them. It will now attempt to get through the day on whatever was left, like a mechanic who arrived at the garage to find someone had stolen half the tools and all the coffee.

What If REM Gets Cut Short?

REM periods get progressively longer toward morning, which means the last cycle or two of your night contain the most REM sleep. This is an unfortunate design choice, because the last cycle or two are precisely the ones people sacrifice when they set their alarm "just an hour earlier." You are not losing an average hour. You are losing the hour with the most REM in it, which is responsible for emotional regulation, memory consolidation, and the general ability to encounter a minor inconvenience without treating it as evidence that civilisation is collapsing.

People who consistently cut their sleep by an hour do not just feel "a bit tired." They feel everything more intensely, remember things less accurately, and have the emotional resilience of a wet paper bag. They then attribute this to the stresses of modern life, which is like blaming the road for a flat tyre you caused by driving over a nail you placed there yourself, every night, on purpose, because you wanted to watch one more episode.

Should I Buy a Sleep Tracker to Monitor This?

You can. They are moderately accurate, occasionally interesting, and will tell you in forensic detail exactly how badly you are sleeping, which is rather like hiring a private investigator to confirm that your diet is poor. The information is correct. It does not, by itself, fix anything. The most reliable test of whether your sleep stages are properly distributed costs nothing: do you wake up feeling rested? If yes, everything is working. If no, the answer is almost certainly more sleep, not more data about your inadequate sleep presented in colourful graphs on your phone, which you are looking at in bed, which is making the problem worse. Put the phone down. Go to sleep. The graphs will still be there tomorrow. So will you, and in better condition. Start by giving your brain the time it needs. The calculator will handle the schedule. You handle the putting-the-phone-down part.