Why does jet-lag recovery lag?

Researchers at the University of Oxford, University of Notre Dame and F Hoffmann-La Roche have identified a mechanism that limits the ability of the body clock to adjust to changes in patterns of light and dark. This in turn reveals why the body is so slow to recover from jet lag.

Nearly all life on Earth has an internal circadian body clock that keeps us ticking on a 24-hour cycle, synchronising bodily functions such as sleeping and eating with the cycle of light and dark in a solar day. When we travel to a different time zone, our body clock eventually adjusts to the local time. However, this can take up to one day for every hour the clock is shifted, resulting in several days of fatigue and discombobulation.

In mammals, the circadian clock is controlled by an area of the brain called the suprachiasmatic nuclei (SCN), which pulls every cell in the body into the same biological rhythm. It receives information from a specialised system in the eyes which senses the time of day by detecting environmental light, synchronising the clock to local time.

Until now, little was known about the molecular mechanisms of how light affects activity in the SCN to ‘tune’ the clock and why it takes so long to adjust when the light cycle changes. So the Oxford University team, led by Dr Stuart Peirson and Professor Russell Foster, used mice to examine the patterns of gene expression in the SCN following a pulse of light during the hours of darkness, intended to simulate jet lag.

They identified around 100 genes that were switched on in response to light, revealing a sequence of events that act to retune the circadian clock. Amongst these, they identified one molecule, SIK1 (salt inducible kinase 1), that terminates this response, acting as a brake to limit the effects of light on the clock. When they blocked the activity of SIK1, the mice adjusted faster to changes in the light cycle.

“If you think about, it makes sense to have a buffering mechanism in place to provide some stability to the clock,” said Dr Peirson. “The clock needs to be sure that it is getting a reliable signal, and if the signal occurs at the same time over several days it probably has biological relevance. But it is this same buffering mechanism that slows down our ability to adjust to a new time zone and causes jet lag.”

Professor Foster, Director of the Oxford University Sleep and Circadian Neuroscience Institute supported by the Wellcome Trust, said a cure for jet lag is still several years away, “but understanding the mechanisms that generate and regulate our circadian clock gives us targets to develop drugs to help bring our bodies in tune with the solar cycle. Such drugs could potentially have broader therapeutic value for people with mental health issues.”

Disruptions in the circadian system have been linked to chronic diseases including cancer, diabetes and heart disease, weakened immunity to infections, impaired cognition and mental illnesses such as schizophrenia, bipolar disorder and depression.

The study was funded by the Wellcome Trust and F Hoffmann-La Roche. It was published in the journal Cell.

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