Michael, 25, has had a long day. For the past 16 hours his body has been operating at full power to meet the challenges of daily life, metabolism humming, heart pumping, neurons firing. Like all bodies, Michael’s accumulates wear and tear just by being active, damage his system must repair overnight during sleep. Of course, Michael isn’t aware of how all the cogs in his body’s machinery are working, or how squeaky they are after a hard day’s work–he just notices that he’s tired, lays down for a night’s rest, and quickly drops off to sleep.
Across the street, Harold is already in bed. He’s had a full day as well, but sleep won’t come as easily for him. Now well into his 60s, insomnia is a regular and unwelcome visitor, one which he often tries to drive off with the help of over-the-counter sleep aids. After an hour or so of tossing and turning, Harold finally enters the first, lightest stage of sleep.
In the meantime, Michael slumbers soundly and has already progressed to the third stage, slow wave sleep. All across his neocortex the neurons are at rest, until a brief flurry of excitement as they fire synchronously, then quiet down again until the next burst comes, rocketing up and down like a rollercoaster. His brain uses this time to recharge its glucose stores, as well as cleaning up waste products like free radicals and amyloid protein (a peptide infamous in plaque form as the hallmark of Alzheimer’s disease). It also prunes away unnecessary connections between neurons and strengthens more important ones. Growth hormone is released during this stage, and perhaps most importantly, short term memories from Michael’s busy day are taken from their holding place in the hippocampus, consolidated, and transferred to long term memory, facilitating learning.
Harold isn’t having quite as good a time of it. He’s still sleeping lightly, and if recent nights are any guide, he may not enter slow wave sleep at all. That sleep aid he took isn’t doing him any favors either–most hypnotic drugs interfere with stage three sleep. When he finally does catch a few of those sweet delta waves, they’re shallower than Michael’s. Folk wisdom holds that older people don’t need sleep as much, but Harold’s brain, being older, has much more need of its repair functions than Michael’s. For starters, Harold’s brain has built up quite a bit of amyloid protein, a problem which will only get worse with age, and may raise his risk of Alzheimer’s disease.
Michael cycles through light sleep, slow wave and finally REM sleep every 90 minutes until he wakes up eight hours later feeling well-rested. But Harold wakes up (for the third time that night) after only six hours. While both men made appointments the previous day for this morning, only one of their brains did the work last night to cause that memory to stick. Michael is on time to see his doctor, but Harold receives a bewildered phone call from the old friend he was supposed to meet for breakfast.
Harold isn’t alone. Sleep quality declines steadily with age, increasing nighttime awakenings and most drastically impacting slow wave sleep. The effect of aging on sleep likely itself accelerates the aging process, causing damage to go unrepaired and waste to accumulate. But despite the importance of this state that we spend a third of our lives in, we still have relatively scant knowledge of its workings and virtually no ability to treat it when it inevitably becomes dysfunctional. Exactly why these changes happen, how they happen and how to put a stop to them are questions shrouded in mystery. Will we have solved them by the time Michael is in Harold’s shoes–along with all the rest of us?