Last week, we began exploring some of the reasons we sleep and the processes that regulate sleep cycles. To examine this more thoroughly, it is important to keep in mind that our sleep drive is lowest in the morning when we wake up; it gradually increases as the day progresses. During sleep, our sleep drive gradually weakens as we “recharge our energy reserve.”
Napping also reduces our sleep drive. The longer we nap the more our sleep drive is reduced. Napping close to bedtime (even brief dozing off while watching TV) weakens our sleep drive just when we most need it.
In some ways the sleep drive is similar to hunger, and napping is like snacking. As grandma used to say, snacking close to meal time may ruin our appetite.
The circadian process: People, like most animals, have powerful internal ‘clocks’ that affect their behavior and bodily functioning, including digestion, body temperature, and sleep/wake pattern. Many of these ‘clocks’ work across roughly 24-hour periods.
For example, if we record a person’s internal body temperature for several days under the same conditions, we will see a consistent up and down pattern across each 24- hour day.
Most peoples’ internal temperature will reach its lowest point around 3 or 4 AM, will rise through the morning and early afternoon, and will hit its peak (highest) point around 9 or 10 PM. Then, their temperature will begin to fall until it hits its low point in the early morning hours, after which it starts rising once again. We tend to fall asleep as our core body temperature is falling and wake up in the morning after our core body temperature starts rising.
We sleep best when our sleep drive is strong and our bedtimes and waketimes are in sync with the internal, biological clock that regulates our sleep and wakefulness. One way to understand how our biological clock regulates sleep is to realize that this clock operates by sending alerting (waking) signals that differ in strength across the 24-hour day and that these alerting signals oppose our sleep drive. You can think of the temperature as an indication of how strong the alerting signal is.
In other words, a higher temperature indicates a higher alerting signal and a lower temperature indicates a weak alerting signal.
Under normal conditions, if there are no sleep problems, the alerting signals sent by our clocks increase from the time we wake up in the morning until a time in the evening when our alerting signals start decreasing. In other words, as our sleep drive increases and promotes sleepiness, the alerting signals from our clock ensures that we do not fall asleep during the day when we need to be alert to carry out our daily activities.
The ideal time to fall asleep is when our alerting signal starts to decrease in the evening and our sleep drive is high. In other words, at a time when the balance between our sleep promoting and alertness-promoting drives is tipped toward sleep. Then after we fall asleep, our alerting signal continues to decrease, which is a good thing because at the same time our sleep drive weakens as well. In that way, the net effect of the sleep promoting and opposing factors continues to be in favor of sleep so that we can continue to sleep.
Then, sometime in the early hours of the night, the alerting signals from our internal clock start to increase again. About 1-3 hours after that, we naturally wake up for the day.
Keeping a very irregular sleep-wake schedule can interfere with our ability to sleep well because it weakens the signals from our circadian clock. Irregular sleep schedules subject our bodies to a frequent ‘jet lag’- like experience, during which we try to sleep out of sync with our biology. To keep your biological clock healthy it is particularly important to keep regular wake and out of bed times. Regular wake and out of bed times mean the clock is getting light signals at the same time every day. This is important because the clock uses light signals to reset itself every day. Therefore, regular wake and out of bed times and hence regular exposure to natural light help keep the clock “ticking” with a strong and regular beat, which supports good sleep.
Sometimes a person’s biological clock is out of sync with society’s typical daily schedule. People who describe themselves as “night owls” may have a delayed circadian clock relative to most other people. This means that the night owl’s alerting signal starts decreasing later than it does for most people. If night people or “night persons” go to bed when most other people do, they are trying to sleep when their alerting signal is still too strong therefore they have difficulty falling asleep. But, if they wait and go to bed later, they fall asleep much faster because their clock’s alerting signals are already weakening.
“Night people” often have difficulty waking up in the morning. This happens because when they try to wake up during the “normal” societal times their biological clocks are not yet generating strong enough alerting signals.
Compared with young adults, older people tend to wake more often during the night and/or wake too early in the morning without having enough sleep to feel rested. As many as 50% of older individuals complain about sleep problems, including disturbed or ‘‘light’’ sleep, frequent awakenings, and early morning awakenings. Indeed, the percent of the deepest sleep stage, stage N3, relative to total sleep time declines with age. Older age may be associated with a weaker signal from the circadian clock that is due, in part, to insufficient light reaching the brain’s clock. Insufficient light signal to the brain may be related to a decrease in time spent outdoors or age-related changes in light receptors in the eye. Because light exposure is an important regulator of the circadian clock, age-associated reduction in light exposure results in a weaker signal from the circadian clock. In addition, some older adults go to bed and wake up earlier than when they were younger, a shift that is influenced by both biology and life style.
We just discussed some of what we know about the science of sleep and wake regulation. Next week, we will discuss several factors that can have an impact on sleep. If you’re looking for some tips on getting better sleep in the meantime, I shared some tips with Reader’s Digest recently that you can get started with.
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