LOS ANGELES, Calif.–The part of the human brain that controls the sleep/wake cycle begins to decline by middle age, perhaps explaining why some older people have difficulty sleeping and adjusting to time changes, according to a UCLA study released today.

“Aging has a profound effect on circadian timing,” said the study’s senior author, UCLA Chancellor Gene Block, a professor of psychiatry and biobehavioral sciences and of physiological science.

“It is very clear that animals’ circadian systems begin to deteriorate as they age, and humans have enormous problems with the quality of their sleep as they age, difficulty adjusting to time-zone changes and difficulty performing shift-work, as well as less alertness when awake,” he said.

“There is a real change in the sleep/wake cycle.”

The paper suggests a primary cause of at least some of those changes is a reduction in the amplitude of the rhythmic signals from the brain’s master circadian clock–known as the suprachiasmatic nucleus, or SCN.

The SCN, located in the hypothalamus, is the central circadian clock in humans and other mammals and controls not only the timing of the sleep/wake cycle, but also many other rhythmic and non-rhythmic processes in the body, Block said.