Sleep is the critical missing ingredient for a health mind and body, yet few of us really understand sleep. We feel tired in the afternoons, lie awake at night, and feel groggy when the alarm goes off in the morning. In this abridged extract from Sleep Sense by Dr Katharina Lederle, you’ll discover the answer to some common sleep questions. Enjoy!

Why do I feel groggy when I wake up in the morning?

The answer to this question is further proof that sleep researchers and chronobiologists love giving technical names to things they observe. Process W, or sleep inertia, is what they call the grogginess most of us experience after waking in the morning.

Sleep inertia is basically a state of low arousal, causing impairment of the psychological rhythms, namely your alertness and mood. What might be happening on a neural-system level is that the transition from sleep to wakefulness isn’t rapid and the boundaries between the two states are fluid and overlapping: one area of the brain might be awake while another might still be asleep, so the tendency to sleep continues despite awakening.

The duration of this grogginess can be hugely different for each of us (the scientific term for this is inter-individual differences), and it can last from one minute to up to four hours. The length tends to depend on your genes, chronotype and how much sleep you had on the previous night/s. Once it has passed, your alertness level increases quite quickly before it then begins to settle. Incidentally, sleep inertia is another reason why it’s recommended that you keep your afternoon nap to 30 minutes or less. If you sleep longer, you’re likely to enter into deep sleep (stage N3).

Not only is it harder to wake up out of N3, it also takes you longer to make sense of the world around you. But if you only sleep for 30 minutes or so then you’ll stay in the lighter sleep stages and come round relatively quickly.

Eating my lunch always makes me sleepy, but why?

This is probably one of the most persistent myths about sleep. It’s so persistent that many different names for this phenomenon have evolved. Scientifically known as the ‘postprandial dip’ (prandial relates to lunch) or post-lunch dip, it’s a spike in sleepiness levels and causes a drop in alertness in the afternoon. As a result, our performance becomes temporarily impaired.

Because of how close this occurs to eating lunch, the general opinion is that it must be caused by the food we eat for lunch. But have you ever noticed a spike in sleepiness levels after eating breakfast or dinner? If the rise in sleepiness is to do with eating food then why doesn’t it happen in the morning or evening? At this point I need you to come on another short science excursion with me … When we discussed the internal clock, we saw that it regulates many, if not all, processes in the body, including the rhythms of alertness, cognition and sleepiness — none of which is constant across the 24-hour cycle.

I want to take a moment to describe it in more detail. There are two peaks in alertness: one in the morning that starts after waking up, once sleep inertia has dissipated; and another in the evening as part of the wake-maintenance zone. Then there are two dips: a big one at night around 2 a.m. to 4 a.m. and another smaller dip in the afternoon, sometime around 2 p.m. to 4 p.m. — usually just after we’ve eaten our lunch. So rather than the post-lunch dip being caused by what you eat (although what you eat can make it worse) it’s actually your internal clock that’s responsible for this dip in alertness and the simultaneous spike in sleepiness.

When is the best time to sleep?

In an ideal world — ideal from a chronobiologist’s point of view — 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 realize is that there’s no one bedtime that’s best for all of us. (I am using bedtime and sleep to mean the same here, but for some people there might be a big gap between their bedtime and the time when they eventually turn off the light and go to sleep.) Equally, there isn’t one wake-up time that suits all of us. Instead, people differ in their sleep timings and these variations are reflected in the common terms ‘larks’ and ‘owls’ used to describe the tendency for an early or later sleep pattern. As you can imagine, different combinations of bedtimes and wake-up times are possible.

For example, you can go to bed early and wake up early, or go to bed early and wake up late. So, perhaps see larks and owls as the two opposite ends of a continuous spectrum of timing types. This is what the term chronotype (chronos = time) refers to, and what chronotype you are depends to some degree on your genes, age and gender. 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.

If you eat very late at night, it will take your body quite some time to digest the meal because the stomach isn’t prepared to deal with food at this time. Usually you would be asleep and food would only arrive in the morning, so it isn’t expecting any. As a result, your sleep–wake rhythm and eating rhythm become misaligned and if you continue to live against your internal clock this can lead to serious physical and mental health problems.

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