In the Northern Hemisphere, the winter months of November to March are cold and characterized by short periods of daylight. Most animals slow down their activities, many plants die or stop growing, and people suffering from seasonal affective disorder (SAD) experience lack of energy, increased sleep, and depression.

Welcome to the world of biological clocks—the internal molecular machines that keep track of time in organisms as diverse as fungi, fruit flies, plants, and mammals. Join us on a brief tour of some of this season's events and the rhythms behind them.

Circadian Rhythms News This Winter

• Bright light has been used as a treatment for winter depression (also referred to as seasonal affective disorder) since the early 1980s. New research suggests that timing the treatment to match an individual's circadian rhythm will significantly improve the light's antidepressant effect.
  
  Terman, J.S., Terman, M., Lo, E.-S. and Cooper, T.B. 2001. Circadian time of morning light administration and therapeutic response in winter depression. Arch. Gen. Psychiatry 58: 69-75.
  
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• A number of genes that control the endogenous circadian timing systems have been identified in organisms as diverse as plants, fruit flies, and mice. The first human circadian rhythm gene was recently identified in a study of a family suffering from a sleep disturbance that causes people to fall asleep and awaken earlier than normal.
  
  Toh, K.L., Jones, C.R., He, Y., Eide, E.J., Hinz, W.A., Virshup, D.M., Ptacek, L.J. and Fu, Y.H. 2001. An hPer2 phosphorylation site mutation in familial advanced sleep-phase syndrome. Science 291:1040–1043.
  
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• Shift workers and travelers complain of gastrointestinal problems, which might be alleviated by careful attention to the timing of meals. Two recently published studies show that feeding rats only during a limited time each day will change the circadian clocks in their livers and other peripheral organs.
  
  Damiola, F., Le Minh, N., Preitner, N., Kornmann, B., Fleury-Olela, F. and Schibler, U. 2001. Restricted feeding uncouples circadian oscillators in peripheral tissues from the central pacemaker in the suprachiasmatic nucleus. Genes Dev. 14: 2950-2961.
  Stokkan, K.-A., Yamazaki, S., Tei, H., Skaki, Y. and Menaker, M. 2001. Entrainment of the circadian clock in the liver by feeding. Science 291: 490–493.