Dr Katharina Lederle is a sleep and fatigue specialist. She has a Masters in biosciences and a PhD in the human body clock. In this extract from her book Sleep Sense, she explains sleep, and answers the most commonly asked questions.
When people hear I’m a sleep specialist, they ask me all sorts of questions about sleep: what is its role and function, what regulates it and how much sleep do we need? The best way to address these questions is to give you an understanding of what normal sleep is, then answer some of the most interesting questions people ask.
Sleep can be divided into two broad states: rapid eye movement sleep (REM), characterised by fast, wake-like waves, and a quieter, non-rapid eye movement sleep (NREM). The repetition of alternate NREM and REM stages results in sleep cycles, each with a duration of 90 to 120 minutes. For the typical sleeper, this equates to four or five cycles per night.
We enter sleep via NREM stage 1. This is a very light sleep: you become drowsy, your eyes move slowly and your muscles start to relax. You may also experience sudden twitches or muscle spasms. These are nothing to worry about.
NREM stage 2 sleep is also a light phase of sleep, though one during which everything’s taken down a notch. We observe a decrease in body temperature and a slowing down of breathing and heart rate.
NREM stage 3 is what is called Slow Wave Sleep (SWS) or ‘deep sleep’. It takes some effort to wake someone up out of deep sleep; breathing, heart rate and blood pressure slow down even more and the body temperature drops further. This is the phase of restoration and it helps the consolidation of fact-based memories such as learning new vocabulary for your German class or events you have experienced that day, for example.
Finally, we come to REM sleep, which gets its name from rapid, rolling eye movements. In fact, your eye muscles are the only muscles you can use during this stage. The rest of your body is almost paralysed. During REM sleep, breathing rate, blood pressure and heart rate are elevated compared to NREM – in fact, they’re more like those of someone who’s awake.
Are certain sleep stages more critical for our health and wellbeing than others?
No, not at all. While the different sleep stages enable different processes, none is more important than the others.
When is the best time to sleep?
In an ideal world, you sleep when your internal clock tells you to. Bedtime is when you naturally feel tired, and wake-up time is when you naturally wake up and feel refreshed. If you follow your circadian clock, then these times should be around the same time every night or day.
What’s important to realise is that there’s no one bedtime that’s best for all of us. Equally, there isn’t one wake-up time that suits all of us.
Sleeping according to your personal internal clock has real health benefits. It helps keep all other behavioural, psychological and physiological processes in your body in sync with each other.
How can you find out what your individual sleep timings are?
You could do the following: take five days’ holiday (or longer if possible), stay at home, and go to bed and wake up as and when your body tells you to. This way you’re simply following your body’s natural rhythm rather than external demands.
It’s important to not drink alcohol or use any devices such as your tablet or smartphone too close to bedtime, as these activities will impact your natural sleep–wake rhythm, which is what we want to find out about. It’s likely that your body will use the first three or four days to recover from any sleep debt you have accrued in the past. After the fourth or fifth night, you’ll know when you sleep best.
The time at which we sleep is highly individual and depends on several factors, including genes, age and sex. While there’s some change during our life, a temporal preference is always there. Given our body clock needs to be kept in sync with the solar 24-hour light/dark cycle, we need to keep our bedtime and wake-up time as regular as possible. If we don’t, the clock gets confused and we may suffer consequences such as lower performance and even increased risk of disease.
How much sleep do I need?
The amount of sleep each of us needs varies from person to person. It depends to some extent on our genes, our age and when we sleep. Similar to the sleep timings it’s important to know what your sleep need is. It’s important not to beat yourself up for what you may think is too little sleep compared to others, or that you don’t increase the risk of developing health issues due to an actual lack of sleep (which, sadly, is more likely to be the case these days).
Many people believe they need eight hours simply because that is what they read and hear about. While this amount might be in the right ballpark for the majority of people, it doesn’t apply to every one of us.
In fact, the range is quite large when it comes to how long people need to sleep to feel refreshed in the morning. About 60 per cent of the adult population need approximately seven to nine hours, and any duration between six and 10 hours can be appropriate.
Apart from age, gender can determine how much sleep people need, with women needing a bit more sleep than men.
Your sleep behaviour changes as you go through life. The hours, the stages and the timing of sleep rarely stay the same.
Changes become most noticeable from your 40s onwards. Your sleep becomes a bit lighter and more fragmented, and so you tend to wake up more during the night. But these night-time awakenings might also be a result of having to use the bathroom, of health issues that include sleep disorders and an inability of the internal body clock to promote continuous sleep. Another reason affecting women is the menopause. Reductions in REM sleep start to occur after the age of 50.
Our sleep pattern also changes, and as we grow older, sleep timings shift back and forth. What it comes down to is how you feel during the day – do you feel refreshed and in a good mood most days, or do you lack energy and feel low? If so, see your GP or sleep specialist. It might also help to review your current lifestyle and the nature of your job, as both can affect your sleep (think of them as superimposing over your biological factors).
Tomorrow: Why do I wake up at night? Why do I feel sleepy after lunch? Why do I get a second wind in the evening?
Dr Katharina Lederle is a sleep and fatigue specialist. She has a MSc in biosciences and a PhD in human circadian physiology and behaviour (the human body clock). She is co-founder of Somnia, an organisation that raises awareness about the importance of healthy sleep. Sleep Sense is available from www.exislepublishing.com, and wherever good books are sold.