Why sleep matters

Sleep is not idle time

Sleep can look like the moment when nothing happens. It is not. The NHLBI, the US health institute that studies the heart, lungs and sleep, describes sleep as central to health: while you sleep, your body works to keep your brain functioning well and to look after your physical health. How you feel when you are awake depends, in part, on what happens during those hours.

That does not mean one bad night is harmful in itself. Almost everyone has nights like that. What research follows is too little sleep over time, which is linked to more health problems and to more difficulty thinking, reacting, working, learning and getting along with others.

What changes in the body

Heart and circulation. When you fall asleep, your blood pressure and heart rate fall, and your heart works less hard than it does during the day. People who sleep too little or wake often during the night may have a higher risk of coronary heart disease, high blood pressure, obesity and stroke. These are associations: they do not say that sleep explains everything, but they show it is part of looking after your body.

Hormones and metabolism. Your body releases hormones at different times of day. In the morning, for example, cortisol rises and helps you wake up. Studies also link too little sleep to changes in the hormones that control hunger, a weaker response to insulin, eating more fatty, sweet and salty food, and moving less.

Your defences. Sleep also affects your immune system, which becomes more active at certain times of day. That is why people who sleep too little may be more likely to catch colds and other infections.

Mind, memory and mood

Sleep helps you learn and form long-term memories. When it is short, or when its quality falls, it becomes harder to stay focused and think clearly. You may have noticed this on a day when a simple task seems to take twice as long.

Mood is part of the same picture. Bad nights tend to shorten your patience, and worry and sadness also disturb sleep. This two-way link is the subject of the piece “Sleep and mood”, which also covers the habits with the most research support.

How much is enough

For adults, experts recommend between 7 and 9 hours a night. Two US sleep societies reached a similar consensus: sleep 7 hours or more, regularly, to support health. An expert review by the National Sleep Foundation suggests 7 to 9 hours for adults and 7 to 8 hours from age 65.

These are guides, and people vary. Sleeping more than 9 hours is not necessarily a problem, and it may help people who are ill or recovering from short nights. Children and teenagers need more.

A practical sign is how you feel during the day. If you sleep enough and still wake up tired, or if poor sleep is already affecting your routine, it is worth talking to a health professional.

What to look for in your records

Logging the time you fell asleep and the time you woke up shows two things memory tends to blur: how much you slept and how your times vary across the week. After a few days, History helps you see that pattern, along with what got in the way each night.

You do not need to get the minute right. An estimate is enough, and the goal is not a perfect night but noticing what helps you. If you like, bring these records to your conversation with your professional.