Ageing, physical fitness and shiftwork tolerance.

The effects of ageing and physical fitness on shiftwork tolerance are reviewed. Ageing is one of the most cited factors decreasing the health of shiftworkers. Although long-term prospective studies on ageing are few, shiftworkers over 40-45 years of age seem to sleep worse after night, but not after morning shifts. Sleepiness after successive night shifts is also decreased by age although older shiftworkers' ability to resist acute sleep loss seems to be even better. The reasons for the altered sleep and wakefulness of older shiftworkers are probably related to changes in circadian rhythms, especially higher 'morningness'. Sleep need may also decrease with age which could explain some of the differences found in sleep length. Physical fitness as a factor increasing tolerance to shiftwork is a recent finding. Although the effects of physical activity on sleep have been studied in detail, the relationship of physical fitness to sleep is still a controversial issue. In shiftworkers, moderate physical training has been shown to increase sleep length and night-time alertness. It has not been shown, however, that exercise would quicken the circadian adjustment to night work. It is recommended that work time arrangements should take account of the older workers' changing personal references. Continuous night work should be voluntary after 40 years of age. Moderate physical exercise a few hours before the main sleep period is recommended.