Jet lag is the bane of many travelers, and similar fatigue can plague people who work in rotating shifts.
Scientists know the problem results from disruption to the body's normal rhythms and are getting closer to a better understanding that might lead to more effective treatment.
New University of Washington research shows the disruption occurs in two separate but linked groups of neurons in a structure called the suprachiasmatic nucleus, below the hypothalamus at the base of the brain. One group is synchronized with deep sleep that results from physical fatigue and the other controls the dream state of rapid eye movement, or REM, sleep.
The ventral, or bottom, neurons receive light information directly from the eyes and govern rhythms in tune with periods of light and dark. The dorsal, or top, neurons do not receive direct light information and so govern rhythms as a more independent internal, or circadian, biological clock.
It turns out that some of the body's rhythms are "more loyal" to the ventral neurons and others are much more in tune with the dorsal neurons, said Horacio de la Iglesia, a UW associate professor of biology.
Normally the two neuron groups are synchronized with each other, but disruptions such as jet travel across time zones or shift work can throw the cycles out of kilter. Deep sleep is most closely tied to light-dark cycles and typically adjusts to a new schedule in a couple of days, but REM sleep is more tied to the light-insensitive dorsal neurons and can be out of sync for a week or more.
"When we impose a 22-hour light-dark cycle on animals, the ventral center can catch up but the dorsal doesn't adapt and defaults to its own inner cycle," de la Iglesia said.
In the laboratory rats he uses for his research, that normal cycle is 25 hours. When the artificial 22-hour light-dark schedule was imposed, he found that the rats' deep sleep, largely governed by light but also a response to fatigue, quickly adapted to the 22-hour cycle, while their REM sleep continued to follow a 25-hour cycle. As a result, REM sleep did not occur in a normal progression following deep sleep.
"We found that after exposing rats to a shift of the light-dark timing that simulates a trip from Paris to New York, REM sleep needed 6 to 8 days to catch up with non-REM, or deep, sleep, the sleep you usually experience in the first part of the night," de la Iglesia said.