Aging and Sleep: What keeps people from sleeping?
It is a common observation, and subjectively experienced by many of us, that the ability to fall asleep easily and get a sufficient night’s sleep both decline with age, beginning as early as our later twenties. At fifty we spend half as much time asleep as we did in our twenties, and our sleep/waking pattern has changed significantly, according to a recent study published in Neuron (“Sleep and Human Aging,” by Bryce A. Mander, Joseph R. Winer and Matthew P. Walker, April 2017).
The older we get, the more often we go from deeper to lighter sleep during the night, the more times we reawaken during the night and the more easily we are awakened by stimuli. Earlier waking and sleeping times are also common with aging. Daytime naps, both voluntary and involuntary, increase with age and reach 25 percent of adults 75 to 84 years old. Naps also reduce sleep time at night.
Some causes for decreased sleep have long been known: obesity, some prescription drugs, urinary frequency and caffeine consumption. Testosterone may also be a factor, since men experience reduced sleep and all sleep-related problems more than women do, while castrated and transsexual men have female-like levels of sleep problems.
In addition to reduced sleep the gender-related problems include reduced periods of Non-Rapid Eye Movement (NREM) deep sleep, also called slow wave sleep. The brain protein galamin is both testosterone-sensitive and involved in sleep, causing the gender difference.
Now the behavioral changes in sleep have been clearly connected with objective and precise electroencephalograph (EEG) measurements, which show that we lose the ability to sleep well, because our bodies become less able to recognize tiredness. This, in turn, happens because the number of nerve cell receptors that sense the chemical markers of tiredness decrease with age. “It’s almost like a radio antenna that’s weak. The signal is there, but the antenna just can’t pick it up,” says Matthew Walker, professor of neuroscience and psychology at the University of California, Berkeley.
The hypothalamus is an important brain center for both falling and staying asleep. The sleep-inducing protein galanin is detected by sensors on hypothalamic neurons, which become scarcer with age. Another sleep-inducing chemical, adenosine, suffers a similar reduced impact with age due to erosion of the relevant receptors on neurons.
Slow-wave activity (SWA) can be reduced by as much as 75 to 80 percent as compared to young adults, especially early in the night. Both the depth and frequency of slow waves, which demonstrate deep sleep, decreases with age as measured by EEG.
Sleep spindles, brief periods of rapidly increasing and decreasing electrical activity between the cortex and the thalamus, also decrease with age, especially later in the sleep cycle.
On the other hand, wakefulness, promoted by the brain chemical hypocretin/orexin is centered in the “arousal system” of the brainstem and part of the hypothalamus. Receptors for this chemical also erode with age.