Thursday, July 2, 2009

Brain Activity & Muscle Force in Morning and Evening Types

A new study measured changes in brain activity and muscle force from morning to night in morning and evening types.

The study involved 18 people. Nine were “evening types” who prefer to stay up late at night and sleep late in the morning; nine were “morning types” who prefer to go to bed early and wake up early. Data were collected four times in one day: at 9 a.m., 1 p.m., 5 p.m. and 9 p.m.

Results show a difference in brain activity between the two groups. In morning types their “cortical excitability” was highest at 9 a.m.; this level decreased throughout the day. The brain activity of evening types was highest at 9 p.m.

The study also tested reflex response and maximum muscle force. Reflex response was tested using spinal-cord stimulation; muscle force was measured by maximum contractions of the calf muscle.

Evening types became physically stronger during the day. Their reflex response improved and their muscle torque increased.

For morning types the results were mixed. Their reflex response improved throughout the day; but there was no change in the muscle force that they generated.

"We are suggesting that morning people may never reach their true maximum performance,” study co-author Dave Collins said in a
University of Alberta article. “Their brain [activity] is going one way and their spinal cord activity is going the other, so it's offsetting. In evening people, both brain and spinal cord are at maximum in the evening, and they get maximum performance at night."

The authors suggest that these results may have implications for people who perform
shift work.

Another recent study found that both morning and evening types performed well when tested 1.5 hours after waking. But the evening types pulled ahead when both groups were tested after being awake for 10.5 hours. They were more alert and had faster reaction times.

Learn more about chronotypes on the Sleep Education Blog.

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