It's like in the summer you have got a full shot of energy while in the winter you would remain inside your den all day long.

Scientists have not known precisely how the brain can make this rhythm switches.

A new Brandeis University study reveals for the first time the mechanism by which the circadian cells in fruit flies work like a network that enables the insects to adapt their behavior according to seasonal changes.

This way, we could understand how mammals, including humans, adjust physiology and behavior to seasonal environmental changes like short winter days and long summer ones.

It was already known that specific brain cells in Drosophila fruit flies model the daily rhythmic behavior linked to 24-hour endogenous clock machinery.

But until now, there were just theoretical mathematical models explaining how fruit flies and other species, including humans, adapt to seasonal fluctuations in day length and temperature.

"In this study we show how the 24-hour intrinsic molecular clock can produce a variable output, so that it fits any seasonal condition," said lead author Dan Stoleru.

"This is especially exciting because it gives us an understanding of how animals extract vital information from the environment to drive innate behavior such as reproduction, migration or hibernation."

Stoleru, a Romanian researcher in the pioneering National Center for Behavioral Genomics lab led by Michael Rosbash, found this property to be determined by an adaptable brain circuit of oscillating neurons that respond differently to various environmental cues.

The same team had already showed that two types of neurons, morning cells and evening cells, tune the circadian time, regulating the activity of the fruit flies in the morning and evening, day in and day out.

Genetic engineering on these cells revealed that the network is dominated by either one group or the other, linked to day length and season.

In the summer, the evening cells run the show; with the shortening days, the morning cells take over.

"Both kinds of clock cells can be masters at different times. The morning cells like darkness and they become the master clocks in winter, and the evening cells process light information, so they become the masters in the summer, though it all may happen gradually. We don't know how the switch from slave to master actually happens, but at some point, one type of cell appears to take full control." said Stoleru.

The researchers are also interested in the link between depression and circadian rhythm.

The Drosophila's homologue of the human gene GSK-3 is a key part of the biochemical pathway signaling the seasonal light changes to the inner clock.

GSK-3 is targeted with lithium, a common medication employed against a number of mood disorders, like seasonal affective disorder, or winter depression.

Phototherapy is also effective in many types of depression.

"The indication that GSK-3 is at the same time a mediator of light effect and a clock molecule in Drosophila raises interesting prospects for understanding the etiology of various mood disorders as well as the mechanisms underlying some of the therapies employed to compensate them," explained Stoleru.