To answer that question, I first need to explain the sleep cycle and the different stages of sleep.
The Sleep Cycle
There are three main stages to sleep: light, deep and dream. Dream sleep is also known as rapid eye movement (REM) sleep. We cycle through these stages throughout the night, drifting off into a few minutes of light sleep before going into deep sleep, then back into light sleep before going into 20-40 minutes of dream sleep. We then go back to light sleep and repeat the cycle, with more deep sleep in the first half of the night and longer REM cycles in the second half. On average it takes about 90 minutes to complete each cycle, but the exact timing varies from person to person.
Light sleep accounts for over 50% of the time we are asleep, but it is not very restorative, so people who spend more time than normal in light sleep do not feel rested. Although they are sleeping, it is not well.
On a good night, we spend about 25% of our time asleep in deep sleep. This is also known as delta wave or slow-wave sleep, as it is when electrical charges are moving through the neurones in the brain at their slowest rate of four cycles per second. During normal waking consciousness, the rate is twelve to thirty cycles per second, these are known as beta waves. In between are alpha and theta waves.
Growth hormone production is at a peak in deep sleep. It is responsible for building muscle, repairing bones and supporting the immune system, among other things. Insufficient deep sleep is associated with physical tiredness the next day, but not sleepiness. Children spend more time in deep sleep than adults as they have more growing to do.
I knew that a drop in body temperature could promote sleep, but I was surprised to learn that during dream sleep our body’s temperature regulation system is suspended! Dream sleep is also a time of minimal muscle activity, except in the eyes.
Insufficient dream sleep leaves us feeling sleepy and has a detrimental effect on mood and cognition. Inadequate dream sleep can also increase pain perception, so just as pain can keep us awake, a lack of dream sleep can make us more sensitive to pain – a viscous circle.
The need for sleep varies from person to person and declines with age. In the day when we are awake, our body is in a catabolic (breaking down) phase, largely due to cortisol, levels of which are at a peak around 6am and then steadily decline to a minimum between around 10pm and 2am. When we sleep at night, our body goes into an anabolic (building) phase, which is why babies need so much of it. Sleep also allows the body to cleanse and repair, which is why adults also need to sleep, albeit less.
This is a rough guide to the amount of sleep required at each age:
- Newborns up to 3 months: 14-17 hours
- Infants aged 4-11 months: 12-15 hours
- Toddlers (1-2 years): 11-14 hours
- Preschoolers (3-5 years): 10-13 hours
- School-aged children (6-13): 9-11 hours
- Teenagers (14-17): 8-10 hours
- Adults (18-64): 7-9 hours
- Older adults: 7-8 hours.
As you can see, it goes down by an hour or two at each life stage, so that an older adult needs half as much sleep as a newborn.
There is huge variation in how we perceive our quality of sleep. A lot of people who think they have a sleep problem, actually do get enough sleep, and some people who don’t think they have a sleep problem, actually do, but it doesn’t seem to affect how they feel or function. If you struggle with insomnia, I can highly recommend Chris Winter’s book The Sleep Solution: Why Your Sleep Is Broken and How to Fix It.