How Sleep Works

A Guided Tour

The Rest of the Story…

We’ve just made it through a complete sleep cycle. It’s been a little over an hour and a half since you first fell asleep, and you’re getting ready to start yet another cycle. Before you wake up in the morning, you’ll likely go through four, five, or six cycles altogether, each lasting from 90 to 110 minutes. Under normal circumstances, the overall makeup of a typical full night’s sleep will look something like this:

The progression of sleep stages in each cycle is usually quite consistent: You generally go in a regular,   Stage 1 — Stage 2 — Stage 3 — Stage 2 — REM   pattern.

The duration of each sleep stage, however, is not uniform; they shift as the night progresses. Deep sleep stage 3 lasts longer at the beginning of the night, and REM stages tend to be shorter. Towards the end of the night, it’s just the opposite: REM stages last longer, and deep sleep stages are shorter. This means that we do more dreaming in the last half of the night than we do in the first. As morning approaches, the sleep cycle consists almost entirely of light sleep and REM.

The makeup of the sleep cycle also changes as we age: children’s sleep consists of larger proportions of both deep sleep (stage 3) and REM sleep than adults’. In fact, newborn babies spend about half their sleep time in REM. This makes sense if you recall the likely association of deep sleep and REM sleep with physical growth and learning. We can link this in our minds with the fact that childhood, all the way through adolescence, is a time of relentless physical change and growth, new experiences and constant learning. But as we mature into adulthood, our sleep requirements change: the proportion of sleep time spent in both deep sleep and REM gradually declines, while the proportion of light sleep (stages 1 and 2) increases.

Nighttime Awakenings

During the night, as you cycle through the sleep stages, every so often you will wake up for just an instant and immediately fall back asleep again. You do this so quickly, and your awakenings are so shallow, that you usually won’t even be conscious of doing it. You may be under the impression that you “slept through the night”, but in fact nighttime awakenings are built right into your sleep architecture. This behavior was probably incorporated into us through evolution, as a means of keeping tabs on our environment — looking out for threats — even as we sleep. You can think of a submarine coming up to periscope depth every now and then to get a quick look at the surface.

With every partial awakening, you will make some change in your body position. In fact, we are said to shift positions a remarkable 30 to 40 times a night. Partial awakenings cause a corresponding rise in your metabolic rate. Disturbed, segmented sleep will actually require your body to use more energy through the night even if you wind up getting about the same amount of sleep.

Waking Up is Hard to Do

Through the first half of the night, your body’s metabolism and core temperature continue to drop until reaching a low point about four hours or so into sleep — by this time your body’s temperature will have dipped almost 2° F below its daytime normal.

In the last hours of sleep, your body starts to prepare itself for the process of waking up. The hormone ACTH (adrenocorticotropic hormone or corticotropin) is secreted by the pituitary gland in increasing levels, its peak timed to coincide with wakeup. ACTH in turn stimulates the release of other hormones such as cortisol. Cortisol is considered a “stress hormone” but here it is acting as an agent of the circadian rhythm, priming the body for that time when it will have to come out of sleep and swing into action.

The dreaded moment takes place a couple of hours later, as night is turning into morning. You’re sailing along in a nice, stage 2 light sleep — then just like that, it’s all brought to an end by the electronic shriek of your alarm clock. The relentless noise is more than enough to penetrate the sensory filter of sleep, quickly (and painfully) restoring you back to conscious awareness. You’re awake! And with a sigh, you drag yourself out of bed and begin your day.

Nobody is at their best first thing in the morning. You feel sleepy and groggy — your limbs are heavy and your head is full of morning fog. Your desire for more sleep is like a magnetic attraction drawing you back to the warm bed. Objectively speaking, your level of alertness after waking is measurably low, your reactions are dull, and your capacity for decision making, poor. This condition is called sleep inertia — an all too descriptive term. The worst of it usually lasts about 10 minutes or so, but the tail-end effects can last as long as an hour. Studies suggest that our capacity for decision-making just after waking up is off by about 50% compared to when we’re wide awake; and still off by 20% half an hour later.

The effects of sleep inertia can be more intense if you are unexpectedly wrenched out of sleep in the middle of the night, if you get woke up right out of stage 3 deep sleep, or if you have been sleep deprived. It can last as little as a few seconds if you wake up as a result of an immediate threat or danger. It can also be effectively counteracted by caffeine. Let’s hear it for caffeine!

Later on: You’re feeling a little more like yourself now that you’re dressed and ready for work, with a little breakfast and coffee in you. And as much as you hated to leave the comfort of your bed, you know that the benefits of those eight hours are still with you. As you slept last night, sleep worked its magic: Your body generated new cells, repaired damaged tissue, and replenished itself with energy, while your brain rewired neurological connections, integrating new information with old, in order to keep your mind razor sharp. You’ve literally been rejuvenated.

Now as your body’s circadian rhythm kicks in, you’re really starting to feel alert and strong. The sun is coming out and it’s going to be a beautiful day. And with the power of a good night’s sleep backing you up, there’s nothing you can’t accomplish!