Understanding Age Related Sleep Disorders

It is commonly understood that the quality of our sleep gets worse as we age, but it does not have to be that way - poor sleep is not a normal part of aging. Most of the ways in which sleep deteriorates over time is related to the medical illnesses that many people acquire as they age, side effects of the medications used to treat those conditions, environmental factors that are hostile to sleep, and bad habits. Before diving into those topics in later articles, this article will briefly review the structure and function of normal sleep and how it evolves as we age.

There are two categories of sleep, REM (Rapid Eye Movement) sleep and non-REM sleep. A common misconception is that REM sleep is “deeper” than non-REM sleep. In reality, the two types of sleep are entirely different. REM sleep, current research suggests, is important for learning and memory.

Dreams occur during REM sleep, many of which involve rehearsing new skills or organizing new learning so it can be recalled later, but every muscle in the body is paralyzed, except the diaphragm, so we continue to breathe, and the muscles around the eyes (hence “rapid eye movement”).

The muscle paralysis is useful, or we would run away from the dinosaur chasing us or strike out at an adversary in our dream, and indeed people with a condition called REM Behavior Disorder do exactly these sorts of things. During REM sleep, heart rate, breathing rate, and blood pressure skyrocket, possibly in response to the content of the dream.

Non-REM sleep has three stages, numbered 1, 2, and 3 (there used to be a Stage 4, but this stage was recently eliminated by a panel of scientific experts), with Stage 1 representing the shallowest stage of non-REM sleep and Stage 3 the deepest and most restorative. These stages are identified by measuring brain waves, and the deeper stages are characterized by an increase in slow brain activity.

The muscles are not paralyzed during non-REM sleep, but generally this type of sleep is more restful and restorative for the body, with the secretion of certain growth hormones and other processes involved in growth, repair, and regeneration of various tissues only occurring during certain stages of sleep. Generally, people cycle through progressively deeper stages of non-REM sleep for about 90-120 minutes before the first REM period, after which the cycle repeats itself with Stage 3 sleep gradually disappearing and REM periods becoming progressively longer (Figure 1).

The most fundamental changes in sleep that happen with healthy aging relate to the relative amounts of each stage of sleep that occur each night. Stage 3, the deepest and most restorative stage of sleep is most abundant in children.

Men see a decline in Stage 3 sleep with age, from about 11% of total sleep time in their 30s to about 5% in their 70s. Women, on the other hand, see a slight increase from 14% in their 30s to 17% in their 70s.

The decrease in Stage 3 sleep in men is accompanied by a parallel increase in Stage 1 and Stage 2 sleep, suggesting that even healthy older men sleep less deeply than younger men.

The overall amount of REM sleep, which generally takes up 18-20% of the night in adults, remains stable from our mid-30s onward. Changes beyond the ones discussed here relate to acquired diseases and other factors that were mentioned at the beginning of this discussion.

Sleep is an important and complicated biological phenomenon, but it does not occur just anytime. We only sleep when we feel sleepy, and normally we only feel sleepy at a certain time each night, assuming we slept normally the night before. The explanation for when and why we feel sleepy has been called the “two process model” (Figure 2) because there are two independent processes that interact to determine when we feel sleepy and when we can sleep.

Process C (Circadian) refers to a rhythmic waxing and waning of body temperature and certain hormones that lead to wakefulness and sleepiness at certain times of day. Many people believe that they feel sleepy after lunch because a significant amount of blood rushes to their stomach and away from their brain, or that something in what they ate, such as tryptophan in turkey, made them feel sleepy.

Looking at Line C in Figure 2, it is easy to see that the biological drive to stay awake begins to decline around 2-4 PM, which is when people encounter the “mid-afternoon dip” and some cultures schedule a siesta, or nap. Notice a second dip at 2-4 AM, a time that has been associated with an increase in auto accidents and other causes of injury and death related to a drop in attention and wakefulness for people who are awake at this time.

The second process, process S (sleep) is related to homeostatic sleep drive, or what many people refer to as “sleep debt.” In short, you begin racking up sleep debt the instant you wake up (7 AM in Figure 2 on Line S) and this sleep debt peaks approximately 16 hours later, when there is a parallel low point in the circadian wakefulness drive (Process C) and a peak in homeostatic sleep drive (Process S) that combine to allow a person to fall asleep. Once asleep, the sleep debt is paid off in the form of Stage 3 sleep, which is why this stage of sleep is seen mostly at the beginning of the night and gradually disappears, and assuming an adequate amount and quality of sleep, is fully paid off by 7 AM the next day, coinciding with the next rise in Process C.

The circadian rhythm is controlled by a hormone called melatonin. You have probably heard of melatonin, which happens to be the only hormone you can buy over the counter. The usual peak of melatonin, which signals the brain that it is time to go to sleep, happens two hours prior to bedtime (so if you’re taking melatonin at bedtime, with few exceptions, you’re taking it wrong!) and is opposed by light, particularly blue light. Since the lens of the eye gets increasingly more yellow with age, a greater amount of blue light is blocked, which is one factor that leads to a weakening of peak melatonin secretion.

Bright light in the evening, as well as exercise and food just before bed or in the middle of the night, can suppress melatonin and interfere with the ability to fall asleep and stay asleep. An irregular sleep schedule can also interfere with the normal circadian ebb and flow of melatonin and produce a feeling like jet lag, which makes sense because travel across time zones that results in light exposure at a different time than we are used to is exactly what causes jet lag.

Now that we have a basic understanding of how sleep functions, what the building blocks of a good night’s sleep are, and how they fit together, next month’s article will begin to look at how sleep can be compromised as we age. Stay tuned, sleep tight, and don’t let the bedbugs bite.

About The Author

Matthew Weiss, M.D., F.C.C.P. is board-certified in internal medicine, pulmonary disease, critical care medicine, and sleep medicine. Dr. Weiss is medical director of the Physician Diagnostics Sleep Program at Harbor Medical Associates, where he diagnoses and treats the full range of sleep disorders. For information on the Physician Diagnostics Sleep Program, or to arrange a consultation call (781) 952-1460 or visit www.harbormedical.com.

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