[Light therapy can restore a disturbed sleep/wake rhythm in dementia: but what is the basis of its efficacy?].

In dementia short nocturnal sleep length, agitation and naps in the day-time suggest a chronobiological disturbance. Bright light therapy seems indicated because of its chronobiological activity. However, dementia is accompanied by a decrease in volume and number of active cells of the suprachiasmatical nuclei, the internal clock. Is the circadian system still functioning then or is it irrevocably damaged? The findings concerning the circadian rhythm of several physiological functions are not consistent but do not show a weakened rhythm. Rest/activity and sleep/wake rhythms, however, are weaker in demented elderly than in their peers. All things considered the internal clock is still functioning and the Zeitgeber maintain their chronobiological activity. Research shows that bright light therapy can restore the sleep/wake rhythm in dementia but research does not uncover the working mechanism. Further studies can give clues for alternative treatments. If the effect of light therapy is based on its Zeitgeber function, other Zeitgeber may be considered as treatments. There is some evidence that an increase in social contacts is effective. The efficiency of light therapy can also be caused by stimulation in a situation of sensory deprivation. There are only anecdotical indications that this stimulus role can also be fulfilled by music.