How to get eight uninterrupted hours – by a sleep professor

The solution to getting the sleep that you need lies in daylight hours, from setting your circadian rhythm to learning not to panic

Our modern fixation with getting eight solid hours’ sleep – you may be glad to hear – is nonsense. Healthy sleep can range from six-and-a-half to 10 hours a night. Teenagers need around nine, but as we age we sleep less. 
Why? I suspect because memory consolidation, and the processing of new information, are two of the main purposes of sleep, and in our more mature years, we simply come across less that’s new to us. 
So, how do you know if you’re getting enough sleep? If you are more irritable, and less empathetic; if your decision-making is erratic and flawed and you oversleep on free days then it may be that the answer to getting the sleep that you need lies in your daylight hours.
All life on earth has its own internal circadian rhythm, synched to the day/night cycle in which we have evolved. In mammals, humans included, it is mostly light that keeps this rhythm in step with our environment. In studies spanning more than 15 years, my team discovered that – in addition to the rods and cones we use to see things – the eye contains a third kind of light sensor that detects these dawn/dusk signals and aligns our sleep-wake cycle to our external 24-hour world. 
These cells are relatively insensitive to light. In the centre of your living room, under electric light, you will be exposed to around 100 Lux (the standard unit of measurement for light level intensity) – not bright enough to align the sleep-wake cycle. Outside, even on a grey day, you’ll get 10 to 20,000 Lux, and this will provide a strong signal. So you really need to get outside, to set your circadian rhythms.
Dusk and dawn light is the most effective for circadian regulation, while the middle of the day is what’s called the dead zone – far less effective. Back when we were all agricultural workers, we got symmetrical dawn/dusk exposure, helping our circadian clocks to stay on track. 
Today, many of us only sample one part of the cycle, upsetting our clocks, negatively affecting our sleep and making us less alert in the day. I think this is the reason why dog owners are reported to get better sleep: you have to walk your dog first thing in the morning and last thing at night.
Morning light advances the circadian clock (making you get up and go to bed earlier), while evening light delays it, (meaning you go to bed and get up later). Fail to get morning light, and you’ll want go to bed later. But if you need to get to work early, you probably won’t get all the sleep you need, leaving you chronically sleep-deprived over time. 
Tempted to sleep in on weekends to catch up? If you do, you’ll miss two mornings of advancing light, making getting up on Monday morning even more difficult. It’s a slippery slope. So, stick to your routine, even on holidays. 
The circadian system regulates our entire biology, even our ability to solve problems. In fact, the data suggests that for adults, 11am to noon is the peak period for problem-solving and alertness, so it’s a good time to knuckle down to hard work. If you’re an adolescent, brain performance peaks later – around 2pm or even later. Studies have shown that exam grades increase if tests are given in the early afternoon rather than first thing in the morning.
A 20-minute nap – but not later than six hours before bedtime – has been shown to make you more effective during the second half of the day. Any longer, and you can go into deeper sleep, recovery from which can leave you groggy, making it counterproductive. Don’t panic if you don’t sleep – rest and relaxation contribute to memory consolidation and brain performance too.
Our sleep is primed by two factors. Our circadian clock and the homeostatic drive for sleep – the process through which our need for sleep builds steadily from the moment you wake. Part of the homeostatic drive for sleep is driven by the build-up of a substance within the brain called adenosine. 
Caffeine blocks receptors in the brain that detect this adenosine, which is why you feel reinvigorated by coffee. But a cup in the late afternoon can delay the onset of sleep, because caffeine is still at high concentrations in the blood up to nine hours later. My tip is to switch to decaf after 2pm.
Eating your biggest meal of the day in the evening is a mistake – our digestive processes slow throughout the day. Trying to sleep while still digesting dinner is likely to lead to stomach problems such as acid reflux. So, as the old proverb tells us, you really should eat like a king at breakfast, a prince at lunchtime and a pauper at dinner.
Most people don’t have a sleep problem. They have a stress or an anxiety problem, which prevents them from sleeping well. So, finding ways to wind down at the end of the day is critical. When the concept of mindfulness first started rattling around, I dismissed it alongside hippy crystal waving. Then I looked at the data and I was converted. 
For many people, it’s a spectacularly useful relaxation technique. I now think some form of relaxation technique should be on the school curriculum. The quality of our sleep defines the quality of our work, even our lives. It couldn’t be more important.
Aside from light, two other things influence whether you’re a lark or a night owl: age and genetics. There’s nothing much you can do to alter the latter, so in a way, your parents are still telling you when to go to bed. If you are a lark, then stop fighting it. Get to bed early. It may not feel very rock’n’roll but there’s nothing glamorous about being crotchety and unproductive during the day.
The last thing we do before bed is stand in the most brightly illuminated household space, looking into an illuminated mirror, potentially raising our alertness. So get a dimmer switch.
A few years ago, a Harvard study found that looking at a light-emitting eBook for four hours on its highest light setting for five consecutive nights delayed sleep by 10 minutes. Statistically significant, but biologically meaningless. It is not the light from your device that stops you from sleeping, it is the anxiety generated by social media, your emails, the news… I’ve stopped listening to the evening news before bed.
We now have very good, clear data showing that waking once or more at night (known as biphasic or polyphasic sleep) is the default state for all mammals. So the key thing, if you do wake, is not to get anxious or assume that’s the end of your sleep. 
Stay calm, and keep the lights low. You may want to leave the bedroom to read a book for a while as a distraction, or my favourite trick, listen to Radio 4 Extra. 
Getting to sleep requires a small but significant drop in core body temperature. Too hot, and you’ll struggle. That temperature tends to be different for men and women, because women tend to be smaller, so lose heat more easily, and have less heat-generating muscle too. Separate duvets are the solution.
As we age we’re more likely to snore because we deposit more fat in our tongues and the muscles of the throat become more relaxed, meaning that our airways can become obstructed or reduce our ability to breathe easily. If earplugs don’t help you cope with your partner’s snoring, then find an alternative sleeping space if you can. 
It won’t be the end of your marriage, but it will be the beginning of a better-rested, less resentful and generally happier one!
Russell Foster is a professor of circadian neuroscience and the head of the Nuffield Laboratory of Ophthalmology at Oxford University
As told to Hattie Garlick
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