CIRCADIAN AND PULSATILE RHYTHMICITY IN THE AGING PROCESS: The aging process produces morphological and neurochemical alterations in the suprachiasmatic nuclei as well as major alterations in the quality of sleep. In addition, aging is frequently accompanied by changes in life style due to different, often less demanding, social and occupational activities, leading to an attenuation of the synchronizing effects of the light-dark and activity-rest cycles. Together, these different elements contribute to a decline in temporal organization in the elderly, a phenomenon which starts in the third decade for some variables. There is a characteristic phase shift with age: in an 80-year-old individual, the circadian cortisol peak occurs about 3 hours earlier than in a 20 year-old-individual. JET LAG AND NIGHT SHIFT WORK: The circadian rhythm and environmental conditions can become desynchronized after transmeridian flights, a phenomenon commonly called jet lag. In night shift workers, such desynchronization creates an important public health problem. The impact may be underestimated since 15 to 20% of the work force in industrialized countries work permanently or occasionally on night shifts. The resulting dissociation between environmental signals and the wake-sleep cycle leads to various health problems. No truly effective therapeutic strategy has been developed although ongoing research, particularly on the use of light and/or melatonin, provides some promising perspectives.