Deep sleep: how to increase it for real.

You increase deep sleep by changing the conditions of the night, not by adding hours in the morning: a cool, dark room, no glass of wine after dinner, your last coffee early, a fixed wake-up time and movement every day. Extra hours on their own mainly lengthen dreaming sleep.

Your watch says 38 minutes of deep sleep and you feel behind. People bring me this almost every week. Before you change anything, let's look at what you are seeing.

How much deep sleep does an adult get?

Less than most people imagine. The night is made of cycles of about ninety minutes, four to six a night. In the first cycles the deep block, or slow-wave sleep, lasts thirty to forty minutes, and then it gets thinner until it almost disappears. Light sleep takes up nearly half the night, and dreaming sleep about a fifth.

• It repairs the body. This is when growth hormone is released: it keeps muscle, bone and skin in good order.
• It washes the brain. The fluid around your nervous system flows more strongly between the neurons and carries away the waste of the day.
• It stores what you learn. Slow waves move what happened during the day into long-term memory.

Why is my deep sleep so low?

Because of your age, the heat and what you do in the evening. None of them is obvious at the time, and all three cut into your deep sleep.

It goes down with the years, and that is normal. Growth hormone falls slowly from your twenties onwards, and the share of deep sleep goes down over the same years. That is why, at fifty, the conditions of your night matter more than at twenty-five.

The rest is up to you, in the last four hours. Alcohol makes you drop off sooner and then breaks up the part that repairs you. A room that is too warm cuts deep sleep short: your body has to cool down to get there. Afternoon coffee can shorten it even when you fall asleep easily, and a large dinner keeps digestion going.

What really increases it

Move every day: this is one of the strongest levers. People who train regularly tend to get more deep sleep and to fall asleep faster: a stronger need for sleep, a temperature that rises and falls, a more flexible nervous system. What matters is that it happens almost every day, not that it is heroic.

Have a hot shower an hour to an hour and a half before bed. It sounds like the opposite of what you need, and that is why it works: hot water widens the blood vessels in your hands, feet and face, so when you step out your body releases heat faster. Your inner temperature drops sooner, and that opens the door to deep sleep.

A cool room, a fixed wake-up time, a sober evening. Set the thermostat between 18 and 20 degrees and take off a blanket: synthetic sheets hold the heat in, cotton and linen let it out. Regular hours make your sleep deeper on their own: same wake-up time seven days a week, ten minutes of daylight once you are up. One glass at most at the table, your last coffee straight after lunch, dinner finished three hours before bed.

Deep sleep comes when you prepare the conditions for it. Pick one thing this week and keep it going until Sunday.