Brain Structures Control Direct Effect Of Light On Behavior

MADISON, Wis. -- The biological clock reminds us to go to sleep every 24 hours, but under certain lighting conditions another internal regulator may be telling us to take a nap. University of Wisconsin Medical School researchers have pinpointed two connected brain structures that control the way light affects rodent sleep activity that's separate from the biological clock.

As reported in the current (July 21) Proceedings of the National Academy of Sciences, the structures are different from the one that controls normal circadian sleep patterns. But they're all part of the visual system, the only route by which information in the form of light is transmitted directly to the central nervous system.

"This finding may help explain how lighting conditions can affect peoples' alertness levels, such as in a darkened lecture hall," said UW Medical School associate professor of psychiatry Dr. Ruth Benca, who directed the research. "It may support the need for different lighting in offices and homes that can help people avoid unwanted drowsiness or insomnia."

Regular patterns of rest and activity are governed by each person's biological clock, and usually coincide with light changes related to the 24-hour night/day cycle. The resulting circadian rhythm makes humans sleep when it's dark and be active in daylight. The opposite effect occurs in nocturnal animals.

But major changes in lighting conditions can override normal patterns associated with circadian rhythm. Previous studies have shown that such lighting changes can produce immediate alterations in sleep/wake patterns in humans and animals.

"This 'masking' of normal circadian rhythms suggested to us that light could be acting directly on sleep and wakefulness centers in the brain without necessarily affecting regions involved with the circadian pacemaker," said Benca. "It hinted that different mechanisms might be involved."

Benca's graduate student Ann Miller set out to learn which components of the visual system might control the effects of acute lighting changes on sleep patterns. Her experiments involved albino rats, whose sleep habits are dramatically altered with lighting changes that are unlinked to circadian timing.

In six rats Miller surgically removed the visual cortex, the portion of the brain responsible for conscious vision; in five others she removed the superior collicullus and the pretectum, tightly interconnected mid-brain structures that control visual attention, orientation, pupil size and eye movements. In all animals the suprachiasmatic nucleus, which controls circadian rhythm, was left intact.

Miller exposed all 11 surgically treated rodents and six normal rats to condensed periods of light that wouldn't affect circadian rhythms, and measured sleep patterns with brain electrodes. The UW researchers found that rats whose mid-brain structures had been removed did not respond to the acute light changes, while normal rats and those whose visual cortices had been removed did.

"We know bright light affects sleep patterns and moods in many people, particularly patients with seasonal affective disorder," said Benca, who treats patients with sleep disorders at UW Hospital and Clinics. "Additional studies should help us understand the role these mid-brain structures may play in light therapy."

Benca is also a clinician/researcher in the UW HealthEmotions Research Institute, which aims to scientifically determine how emotions influence health.

Collaborators on the study include William Obermeyer, scientist in the psychiatry department, and Mary Behan, professor of veterinary medicine.