While a restful night’s sleep is tougher to achieve as we get older, age has little to do with it.

“If you look at older healthy adults, they have no complaints,” said Dr. Barbara Parry, associate professor of psychiatry at U.C. San Diego’s School of Medicine.

Still, almost 40 percent of people 60 or older have trouble sleeping. The reason in most cases is ill health.

“As we get older, we develop medical problems, whether it’s heart disease, neurological or arthritis,” said Sonia Ancoli-Israel, a professor of psychiatry at UCSD. Any medical or psychiatric illness can cause sleeping difficulty, she said.

And if illnesses don’t keep us awake at night, the medications we take for them often will, she said.

Then there is the duration of sleep itself.

Sleep comes in four stages, the deepest being the last stage. The problem, sleep-disorder specialists say, is that the final stage begins to diminish in our 20s. By the time we reach our 50s, “there’s not a lot of sleep left. So noise that may not have bothered us when we were younger now disturbs us,” Ancoli-Israel said.

Obesity also can contribute to insomnia. Sleep apnea, a condition in which breathing is stopped temporarily while a person is sleeping, is directly related to being overweight.

Cramps and muscle spasms that cause kicking and jerking legs and arms during the night also get worse as one ages.

And there is plain old stress, which exacts a much heavier toll on the body later in life than in one’s 20s, 30s and even 40s.

Take all of this and combine it with the natural change in one’s circadian rhythm, or internal body clock, and it is a wonder we get any sleep at all.

While ill health contributes to insomnia, the most common reason we wake up in the middle of the night is that our body no longer may be accustomed to falling asleep at its usual time.

“Most adults get sleepy around 10 or 11, sleep seven to eight hours and wake up,” said Ancoli-Israel, who is also director of the Sleep Disorders Clinic at the Veterans Affairs San Diego Healthcare System. “Part of that is controlled by our core body temperature, which drops at night when we get sleepy and rises in the morning when we get up. As we get older, the whole rhythm changes.

“I hear complaints all the time from people who say, ‘I’m waking up in the middle of the night.’ The problem is that their body temperature is rising at that hour, and, as far as the body is concerned, it is time to get up. If they went to sleep earlier and got up at three or four in the morning, they’d be fine. But that doesn’t fit in with our society.”

People tire earlier as they age, often finding themselves dozing off in front of the television only to wake up when it’s time to go to bed.

And there’s the rub. Suddenly, a person is wide awake. So what to do? Some keep the TV or radio on to lull them to sleep. And while that may work at first, the intermittent sounds and flashing sights serve to wake people when they should be sleeping.

Exposure to bright light in the evening also delays the circadian rhythm, pushing sleepiness further into the night or early morning, Ancoli-Israel said.

She said that while many baby boomers rely on substances such as alcohol to make them sleepy at night, “the alcohol leaves the bloodstream and they’re wide awake” when they are ready to go to bed.

People’s sleep patterns have adjusted over time to suit evolving lifestyles.

“That changed our chemistry over time, and I think we paid a price for it,” said Barbara Parry, who also serves as director of research at the UCSD’s Women’s Mood Disorders Clinic. “As people age, it takes longer for them to recuperate from sleep deprivation.”

Staying in bed to compensate after waking up won’t work, both professors say.

“The more time you spend in bed, the more fragmented your sleep becomes,” Ancoli-Israel said. “Eight hours’ sleep out of eight hours in bed is a lot more efficient than eight hours’ sleep and 10 hours in bed.”

For a while, the consensus was that people needed less sleep as they got older. Today, Parry said, “the evidence is not as strong for that.”

There is evidence that too much sleep actually can be detrimental.

“People who had more than nine hours had an increase in early mortality and a shorter life span,” said Parry, adding that the comfort window seems to be seven to nine hours.