When Carol Gee turned 55, she made a new friend: 4 a.m. In the past, she usually slept through it. But once she entered menopause, it became her new wake-up time. “I would go to sleep and wake up every morning around the same time, almost like you set the clock,” says Gee, who is now 68.

She’s not the only older adult to have experienced an exasperating shift in her sleep cycles. In 2017, a national poll conducted by the University of Michigan found that 46 percent of adults 65 and older have trouble falling asleep on a regular basis.

As people age, the body changes in all sorts of predictable ways. Joints stiffen. Brains can slow. Wounds take longer to heal. And sleep patterns shift, too. This can come as news to many, says Michael V. Vitiello, a psychologist at the University of Washington who specializes in sleep in aging.

The most noticeable — and often most aggravating — changes are how sleep and wake-up times change and sleep gets lighter, often beginning in middle age. Gone are weekend snoozes to 11 a.m. and the ability to sleep through a noisy garbage truck down the block.

The most common shift is a tendency to rise with (or before) the birds. Circadian rhythm researchers call it “morningness,” and have found that, not surprisingly, it tends to happen as people’s preferred bedtime skews earlier with age. Scientists have documented the changes in circadian rhythms that occur with aging, but they are still learning why they occur, Vitiello says.

Sleep architecture — the stages and depth of sleep — also changes with age. Older adults take longer to fall asleep, and they wake up more often. They tend to linger in the deepest phases of sleep for less time than younger adults, and they get less rapid eye movement sleep, too. While the exact purpose of REM sleep is still unclear, it appears to be important for memory and learning. Less restorative sleep at night can lead to a tendency to nap during the day. (As long as naps aren’t so long that they interfere with falling asleep at night, they’re considered part of a normal sleep pattern.)

But not every restless night is benign. Studies have found that poor sleep can pose a particular threat to older adults: Falls, depression and anxiety, problems with memory, and increased suicide riskare among the impacts of sleep issues in this population group that researchers have found.

Some sleep disorders — REM sleep behavior disorder in which people act out vivid dreams, circadian-rhythm sleep-wake disorder, which causes a mismatch between a desired bedtime and the body’s interest in sleeping, and sleep apnea, which causes breathing to stop repeatedly during sleep — have been associated with increased risk of Parkinson’s disease, Alzheimer’s and dementia.

But as with changes in sleep architecture and timing, scientists are still unsure why those risk associations exist. Kristine Yaffe, a neuropsychiatrist at the University of California’s San Francisco School of Medicine who specializes in dementia, warns that there are more questions than answers when it comes to dementia and sleep.

“If people have dementia, they tend not to sleep well,” she said. “Do sleep problems actually lead to dementia? Sometimes that’s hard to tease out.”

In a 2017 study in the journal Sleep, researchers found that people with disrupted sleep had a 1.68 times higher risk of cognitive impairment and/or Alzheimer’s than those without sleep disruptions. But as the researchers made clear, there was no way of knowing whether the sleep disorders are a cause of dementia or just a symptom of dementia.

What is clear is the association between good sleep and psychological well-being in older adults. A 2010 study documented that association when it came to sleep quality, but sleep quantity didn’t show the same effects.

And therein, experts say, may lie the key to understanding sleep as you age. If you’re sleeping less, but don’t feel negative effects out of bed, the changes you notice may just be normal age-related. If you experience a sudden change in your sleep, or poor or inadequate sleep is interfering with daily life, altering your personality, or your bedmate says you stop breathing when you snore, that could be a sign of something more serious and it’s worth visiting your doctor.

Sometimes figuring out sleep is just trial and error, however. Florine Salierno, 81, found that years of trying over-the-counter products such as melatonin did nothing for her insomnia, and she didn’t like the various medications she’d been prescribed. Now, she follows her doctor’s suggestion of meditating when she can’t get back to sleep, which she says sometimes helps. Salierno can feel annoyed or even depressed by a sleepless night but mostly she tries not to let it bother her.

Which matches up with what researchers say. Our bodies change over the years and it’s normal for sleep to change, too.

“I think of it as tissue over time,” says Vitiello. “Can you run the 100-yard dash or the 100-meter dash as fast as you could when you were 18?”

Over time, he says, sleep patterns simply change. “A lot of older adults recognize that they don’t sleep the same as they did when they were 18, but they can still function and they’re okay. And all is well with the universe.”