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 risk are among the impacts of sleep issues that researchers have found in this population group.

Some sleep disorders 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.