In part two of a series about sleep, specialist Dr Katharina Lederle answers common questions about sleep and wakefulness and what changes as we age. This is an extract from her book Sleep Sense.
Why do I wake up during the night?
We sleep in cycles and we naturally wake up several times during the night. Usually these awakenings are so brief we don’t remember them and we just shift position in the bed. It’s believed that these awakenings have evolved to allow us to check our environment for danger – something could be out there getting ready to eat us. Waking during the night is only a problem if you wake and struggle to get back to sleep and then feel unrefreshed the next day.
Why do I feel groggy when I wake up in the morning?
The answer to this question is further proof that sleep researchers 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 is that the transition from sleep to wakefulness isn’t rapid … 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 and it can last from one minute to up to four hours. The length tends to depend on your genes 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 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. Why?
This is probably one of the most persistent myths about sleep. The post-lunch dip is 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 …
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 between 2am and 4am and another smaller dip in the afternoon, sometime around 2pm to 4pm – 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.
Why do we experience this afternoon dip in alertness? Originally, it may have developed in response to the midday heat in the African savannah where humans are believed to have first evolved. Think of the siesta time in warmer climates — taking a mid-afternoon nap gets us out of the sun and heat for a while. And, if we look at other species, we see a similar behaviour to getting through the hottest part of the day.
Why do I get a second wind in the evening?
As weird as it may sound, this experience is a scientific phenomenon. While the circadian clock sends out its sleep-promoting signal during the night, quite the opposite happens during the day. Here the circadian clock sends out its wake-promoting signal to oppose the increase in need for sleep across the day.
At the end of a day, sleep pressure will reach its highest point, while in turn the circadian clock’s wake-promoting signal is at its highest to prevent us from falling asleep too early. What you then experience is a sudden and very strong bout of energy a few hours before your bedtime.
We are only starting to fully understand why this happens, but if we think about it in evolutionary terms, it makes sense. Historically, we would have had to have everything ready and in place before nightfall. As we can’t see well at night, being out in the dark to get things organised would have put us in danger of being eaten. A sudden spike in energy levels (or ‘a second wind’) momentarily counteracts the increase in sleep pressure … and allows us to prepare and get our sleeping place ready. Interestingly, this spike in activity can also be seen in other diurnal animals (those that are active during the day).
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. The above is an edited extract from her book Sleep Sense, which is available from www.exislepublishing.com and wherever good books are sold.