Sleep stages and sleep cycles

It’s late, and you’re getting ready for sleep. You lie down in bed, close your eyes... and before you know it, the alarm wakes you up. You know you slept through the night; you may even remember some dreams you had. But what actually happens when you sleep?

When you first go from being awake to asleep, your body relaxes. Your breathing stays the same, but your brain activity slows down slightly. Your heart rate and temperature decrease slightly, your eye movement comes to an almost complete stop. This period of light sleep lasts around 15 minutes. You are most easily awakened during this time.

This is stage 1 of sleep. After this first stage, your sleep continues to be fairly light, but waking up becomes more difficult. Your breathing slows down, your heart rate decreases, and your muscles relax further. Your brain activity is now much slower than during wakefulness, but there are short bursts of high activity. This period, stage 2, lasts approximately 20 minutes.

Next, your eye movement stops completely and your body prepares for deep sleep. This stage 3 lasts around 20 minutes and is quite similar to stage 2. Now it’s around an hour since you fell asleep. Your brain and body enter deep sleep. Your brain waves and activity are so slow at this point we refer to this as slow wave sleep.

Your heart rate, body temperature, and blood pressure are at their lowest point, and your body regenerates during this time. If you are prone to sleepwalking, sleep—talking or night terrors, they happen now. This is stage 4 of sleep. Waking you up at this point would be hard work. But if you are awoken, you’ll probably feel quite groggy!

After around 30 minutes of stage 4, something curious happens. Your eyes suddenly start moving rapidly — you start rapid eye movement or REM sleep. During REM sleep, your body is completely relaxed, almost paralysed. But your brain becomes as active as if you were awake! This high activity is often associated with dreams.

Some scientists think we would act out these dreams if our bodies weren’t so relaxed and immobilised at this stage. Now we’re up to around 90 minutes of sleep. But you usually sleep for much longer than that a night! This is because the stages repeat in cycles over the course of the night. During the first sleep cycle, you sleep through all five stages, with REM sleep lasting only around 10 minutes.

The second sleep cycle usually starts with stage 2, since you’re already asleep and don’t need to go through the process of falling asleep again. REM sleep is longer this time, and gets progressively longer throughout the night. This is why you dream more towards morning! During the last cycle, you might wake up at any stage. If you wake up during REM sleep, you are more likely to remember your dreams.

If you wake up during a deep sleep stage, you probably feel tired. Sleep cycles change throughout our lives. Children spend a lot more time in REM sleep. Older people spend more time in the light sleep of stage 2. Lifestyle choices, such as exercise and diet, can also have an impact on sleep cycles.

Everyone’s sleep is slightly different. And there is still so much about sleep that we don’t know! If you want to know more about your sleep, keep track of when you go to bed, how energised you wake up, or what you remember of your dreams.