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Source: Lolostock/Shutterstock

The alarm rings, you awaken, but you are still drowsy. Why? Being sleepy in the morning does not make any sense: After all, you have just been asleep for the past eight hours. Shouldn’t you wake up refreshed, aroused, and attentive?

No, and there are a series of ways to explain why (assuming that you do not suffer from either narcolepsy or sleep apnea).

The neurobiological answer

During the previous few hours before waking in the morning, you have spent most of your time in REM sleep, dreaming. Your brain was very active during dreaming and consumed large quantities of the energy molecule ATP. The “A” in ATP stands for adenosine. The production and release of adenosine in your brain is linked to metabolic activity while you sleep, and there is a direct correlation between increasing levels of adenosine in your brain and increasing levels of drowsiness. Why? Adenosine is a neurotransmitter that inhibits (turns off) the activity of neurons responsible for making you aroused and attentive. You wake up drowsy because of the adenosine debris that collected within your brain while you were dreaming.

Who did you sleep with last night?

Couples sleeping in pairs were investigated for sleep quality — for the correct balance of non-REM and REM sleep, as well as their own subjective view of how they slept. For women, sharing a bed with a man had a negative effect on sleep quality. However, having sex prior to sleeping mitigated the women’s negative subjective report, without changing the objective results — her balance of non-REM and REM was still abnormal.

In contrast, the sleep efficiency of men was not reduced by the presence of a female partner, regardless of whether they had had sexual contact. In contrast to the women, the men’s subjective assessments of sleep quality were lower when sleeping alone. Thus, men benefit from sleeping with women but women do not benefit from sleeping with men, unless sexual contact precedes sleep — and then their sleep still suffers for doing so.

Did you go to bed late last night?

People who prefer to stay out late tend to get up at a later time and perform best, both mentally and physically, in the late afternoon or evening. Evening-type individuals were significantly more likely to suffer from poor sleep quality, daytime dysfunction, and sleep-related anxiety as compared with morning-type individuals. Even more disconcerting is that late bedtime is associated with decreased hippocampal volume in young, healthy subjects. Shrinkage of the hippocampus has been associated with impaired learning and memory abilities.

Did you go to bed hungry last night?

What you eat before bedtime might also improve your chances of getting a good night’s sleep. A recent study suggests that eating something sweet might help induce drowsiness. Elevated blood sugar levels have been shown to increase the activity of neurons that promote sleep. These neurons live in a region of the brain that lacks a blood–brain barrier; thus, when they sense the presence of sugar in the blood, they make you feel drowsy. This might explain why we feel like taking a nap after eating a large meal. This is just one more bit of evidence demonstrating your brain’s significant requirement for sugar in order to maintain normal function.

With aging, normal sleep rhythms become increasingly disrupted, leading to daytime sleepiness.

What if you do not get enough sleep?

Although scientists have not discovered precisely why we sleep, they have discovered that we need between six and eight hours every night. Not getting enough sleep makes us more likely to pick fights and focus on negative memories and feelings. The emotional volatility is possibly due to the impaired ability of the frontal lobes to maintain control over our emotional limbic system. We also become less able to follow conversations and more likely to lose focus during those conversations. Sleep deprivation impairs memory storage and makes it more likely that we will “remember” events that did not actually occur. Extreme sleep deprivation also may lead to impaired decision-making and possibly to visual hallucinations. Not getting enough sleep on a consistent basis places you at risk of developing autoimmune disorders, cancer, metabolic syndrome, and depression. Why? Some recent studies have reported that sleep is important for purging the brain of abnormal, and possibly toxic, proteins that can accumulate and increase the probability of developing dementia in old age.

Whatever you are doing right now, stop and go take a nap. Preferably alone.

Gary L. Wenk, Ph.D. is the author of The Brain: What Everyone Needs to Know (2017) and Your Brain on Food, 2nd Edition, 2015 (Oxford University Press).

© Gary L. Wenk, Ph.D.

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