Some 20 percent of people in the US report having experienced fatigue intense enough to interfere with living a normal life. This hits us in our pockets, too: workers who are unproductive because of fatigue cost US employers more than $100 billion a year.
It’s perhaps surprising, then, that we are only now beginning to work out what fatigue actually is. Until recently, daytime tiredness was presumed to be nothing more mysterious than simple physical exhaustion or feeling the need to sleep — the US Centers for Disease Control and Prevention estimates that 35 percent of people are short on sleep.
Combine that with the fact that tiredness is subjective and therefore difficult to measure, plus the subject falls somewhere between studies of the body and mind, and it’s small wonder fatigue has largely escaped scientific scrutiny.
Ancient remedies including adaptogenic herbs have attempted to address chronic fatigue as a nonspecific enhancement of the body’s ability to resist a stressor.
Since tiredness accompanies so many common diseases, not to mention ordinary aging, a better understanding of its causes could improve quality of life for pretty much everybody. A handful of researchers are now trying to figure out the causes, and possible fixes. Although it’s early days, a few clues are emerging.
One cause, we might think, is that life is more exhausting than it has ever been. Caught between the competing demands of work and family, not to mention the ever-present buzz of smartphone notifications, it is no surprise so many of us feel as if we are running on empty. Yet this may be a fallacy.
According to Anna Katharina Schaffner, a historian at the University of Kent in Canterbury, UK, and author of Exhaustion: A history, people through the ages have consistently complained of being worn out, and harked back to the relative calm of simpler times. Over the centuries, fatigue has been blamed on the alignment of the planets, a lack of godliness and even an unconscious desire to die, says Schaffner. “Freud argued that a very strong part of ourselves longs for a state of permanent physical and mental rest,” she says.
In the 19th century, a new diagnosis appeared: neurasthenia. The American physician George M. Beard claimed that this condition, supposedly caused by exhaustion of the nervous system, was responsible for physical and mental fatigue as well as irritability, hopelessness, bad teeth, cold feet and dry hair.
Beard blamed neurasthenia on the advent of steam power and newfangled inventions such as the telegraph. Women’s education was also considered to be tiring for all concerned, while the advent of the printing press brought an abundance of newspapers and magazines to keep up with.”Beard feared that the modern subject was unable to cope with such chronic sensory overload,” says Schaffner.
If modern life isn’t to blame, another possibility is that at least some fatigue is down to a lack of sleep. Researchers distinguish between the need for sleep and fatigue, however, considering them to be closely related but subtly different. The good news is that there is a fairly easy way to tell which might be wearing us out: the sleep latency test.
Used widely in sleep clinics, it is based on the idea that if you lie down somewhere quiet during the day and fall asleep within a few minutes, then you are either lacking sleep or potentially suffering from a sleep disorder. If you don’t drop off within 15 minutes or so, yet still feel tired, fatigue might be the problem.
So if it’s not the same thing as sleepiness, what is fatigue? Mary Harrington, a neuroscientist at Smith College in Northampton, Massachusetts, is one of a handful of researchers looking for a telltale biological signal of fatigue. So far no single marker has emerged that tallies with how tired people say they feel, but “we do have some candidates”, she says.
One possibility Harrington is investigating is that daytime fatigue stems from a problem with the circadian clock, which regulates periods of mental alertness through the day and night. This regulation falls to the brain’s suprachiasmatic nucleus, which coordinates hormones and brain activity to ensure that we feel generally alert by day. Under normal circumstances, the SCN orchestrates a peak in alertness at the start of the day, a dip in the early afternoon, and a shift to sleepiness in the evening.
The amount of sleep you get at night has little impact on this cycle, says Harrington. Instead, how alert you feel depends on the quality of the hormonal and electrical output signals from the SCN. The SCN sets its clock by the amount of light hitting the retina, so that it keeps in line with the solar day. Too little light in the mornings, or too much at night, can disrupt SCN signals, and either can lead to a lethargic day. “I think circadian rhythm disruption is quite common in our society and is getting worse with increased use of light at night,” says Harrington.
If you spend the day feeling as if you have never quite woken up properly but are not sleepy at bedtime, a poorly calibrated SCN might be to blame, says Harrington. She suggests trying to spend at least 20 minutes outside every morning and turning off screens by 10 pm to avoid tricking the SCN into staying in daytime mode.
Another way to reset the SCN is to exercise, Harrington suggests. Several studies have linked exercise — whether a single bout or regular physical exertion — to reduced fatigue. “People with fatigue hate to hear this, but exercise can make a big difference,” she says. This may explain why people who start exercising regularly often report sleeping better, when some studies show they don’t actually sleep for any longer. Quality of sleep may be more important than quantity.
As well as resetting the SCN, exercise fights the flab, and there are good reasons to think that reducing fat levels could help tackle fatigue. Body fat not only takes more energy to carry around, but releases leptin, a hormone that signals to the brain that the body has adequate energy stores.
Studies have linked higher leptin levels to greater perceived fatigue, a finding that makes perfect sense from an evolutionary perspective: if you aren’t short of food, you don’t need the motivation to go out and find some. Interestingly, people who fast regularly often report feeling more energetic than when they ate frequently.
With obesity on the rise, leptin signalling might well be a common reason for feeling tired all the time. But there could be something else at play. People who carry excess fat also show higher levels of inflammation, a part of the body’s immune response that rouses other parts into action by releasing proteins called cytokines into the bloodstream.
Body fat stores large quantities of cytokines, which may mean that more end up circulating, too. As well as stimulating the immune system, cytokines also make you feel drained of energy, as anyone who has ever had a common cold can attest. In 1998, Benjamin Hart at the University of California, Davis, argued that this feeling is an evolved strategy to help fight a bacterial or viral attack: when you need time to rest and recuperate, fatigue is your friend.
Animal studies have shown this effect in action. In one, Harrington gave mice a drug that causes low-level inflammation. She found that while they still moved around their cages and ate as normal, they avoided the running wheels. Contrast that with healthy mice, which seem to seek out the wheels for kicks. “It is like wanting to go out and be active, and have fun and do something that is not necessary for just staying alive.” If low-grade inflammation robs mice of their zest for life, there’s no reason, she thinks, to suspect something similar shouldn’t hold for people.