“I don’t have the energy I used to.” It’s the nearly universal lament of everyone I know over the age of say 35 — patients and friends alike. Here, an excerpt from Harvard Medical School’s Special Health Report called Boosting Your Energy explains why we feel so much more tired as we age. 

Although increased fatigue is not inevitable with increased age, there are certain age-related factors that make you feel weaker and, in general, less energetic. For one thing, your circadian cycle advances, making you want to fall asleep earlier at night and wake up earlier in the morning. Indeed, the most important influence on sleep rhythms is aging.

Insomnia becomes more common as people get older. For reasons that are not understood, older people spend less time in deep sleep, the type of sleep that is considered the most important for restoring your energy. With less deep sleep, you wake up more often in the middle of the night. And the more often you wake up at night, the less rested you feel the next day. The amount of deep sleep that people get each night at age 30 is about half the amount they get at age 20. After age 30, the decrease in deep sleep and the increase in nighttime awakenings continue. Not only do people wake up more often in middle age, it takes longer to fall back to sleep. By age 65, people spend less than 5% of sleep time in deep sleep, compared with about 20% when they were in their 20s.

In addition, melatonin levels decline with age and virtually disappear by old age. Because melatonin helps you feel sleepy at night, its decline can make it harder to fall asleep. The reductions in melatonin and in deep sleep help explain why insomnia becomes more common with age.

As you get older, you’re more likely to make up for your nighttime sleep deficit by napping. But napping can make insomnia worse by keeping you from feeling tired enough at bedtime.

Aging brings a number of physical changes. In women, the onset of menopause creates changes that may cause fatigue. As estrogen levels decline, many women have hot flashes, which can interrupt sleep and even lead to chronic insomnia.

In both men and women, muscle mass declines steadily. By age 70, you’ve lost 30% of the muscle mass you had at age 20. A drop in muscle mass means a decrease in strength and an increase in fatigue. As muscles shrink, they become fatigued more rapidly. With age, your ligaments and joints become stiffer, so you move more slowly. To a large extent, you can compensate for these changes by exercising regularly to maintain strength and flexibility. Weight training increases muscle strength, and stretching increases flexibility.

Along with a decline in physical energy, many people find that their mental energy falls somewhat as they age. They have a harder time concentrating and remembering things, and it takes longer to learn new information. In part, these difficulties reflect age-related chemical changes in the brain that affect memory and learning. On the bright side, staying active mentally by reading, doing crossword puzzles, and learning new mental skills can help offset this age-related decline.

Illness. Aside from normal age-related changes, illnesses that become more common with age can deplete your energy. Many illnesses can interfere with sleep. For example, people with congestive heart failure may awaken feeling short of breath because body fluids accumulate in the lungs while they’re lying down. People with heartburn often find that their discomfort is greatest when they lie down and stomach acids back up into the esophagus. And hundreds of prescription and over-the-counter drugs can make it harder for you to sleep at night.

Fatigue is also a common symptom of a wide range of diseases, including anemia, kidney disease, multiple sclerosis, diabetes, and cancer. In some cases, such as in cancer, fatigue may be the earliest sign that something is wrong. If you suddenly feel listless, it’s worth visiting your doctor to make sure nothing serious is wrong.

Many people find that life only grows more complicated and demanding with age. This can be true both at home and at work.

Caregiving. With people living longer, many middle-aged and older adults find themselves providing regular care to a friend or relative who needs help mowing the lawn, grocery shopping, going to doctors’ appointments, or attending to personal needs. Although such care is usually extended with love, it can cause significant fatigue for the caregiver.

Overwork. During middle age, adults might feel as though they’re in a state of perpetual exhaustion for yet another reason: their busy schedules. Overwork is one of the main causes of ongoing fatigue.

The people hardest hit are typically those of the “sandwich generation” — middle-aged folks sandwiched between the demands of caring for young children and aging parents as well as, in many cases, the challenges of a full-time job.

People of all ages can benefit from the energy boosting tips discussed previously in this blog.

What do you do to keep your energy going as you get older?

Julie K. Silver, M.D., is an assistant professor in the Department of Physical Medicine and Rehabilitation at Harvard Medical School. She is also the Chief Editor of Books for Harvard Health Publications.