Night work is defined as any work performance between the hours of 00:00 and 6:00. Night work hours are generally recognized as most wearing and stressful for any individual.
Man, under natural conditions, is a diurnal species. Exposure to light, in fact, inhibits the secretion of melatonin, the hormone that regulates the sleep-wake rhythm, by the pineal gland: secretion is highest in the middle hours of the night and lowest in the daytime. It goes without saying that exposing oneself to artificial light conditions the secretion of the hormone by "simulating" for the body a daytime rather than nighttime schedule, and this can lead to alterations in the sleep-wake rhythm.
Night work inevitably leads to exposure to artificial light and thus to alterations in the worker's circadian rhythm, which in extreme cases can even go as far as complete inversion (i.e., sleeping during daylight hours and being awake at night).
Alteration of the sleep-wake rhythm causes:
The actual increase in the frequency of accidents at night is still a debated topic but it is still interesting to note that famous accidents caused by human error in our recent history have occurred at night, citing for example that of the nuclear power plants at Three Mile Island and Chernobyl.
Night work also leads workers to follow behaviors that are harmful to their health: it has been seen that shift workers are more prone to caffeine abuse and smoking habits; they are also prone to poorer quality food both due to the closure of eating establishments (cafeterias, bars, diners) and the natural inability to consume full meals equivalent in quantity and quality to those of daytime hours.
The psycho-social aspect of night shifts is also not negligible, i.e., the inability of some workers to cultivate social relationships in a satisfactory and effective manner, a condition that can lead to the development of anxiety and depression that can worsen quality of life and work.
In the long term, the effects of the conditions already mentioned can lead to increased cardiovascular risk (and thus increased likelihood of hypertension, heart attacks, and strokes), eating disorders (overweight, obesity), increased cholesterol levels, increased risk of type 2 diabetes, depressive syndromes, increased risk of addictions, and increased gastrointestinal tract disorders (e.g., gastric ulcers). On the other hand, it is not clear whether there is an increased risk regarding the development of cancer diseases (on the understanding that habits such as smoking are risk factors for lung cancer).
As age in itself is a risk factor for many of the clinical conditions just mentioned, and as a factor on which there is no action possible, it is apparent that the worker over 50 is a naturally more susceptible individual to these clinical conditions.
There may be, on a medical basis, specific exemptions night work that, while not affecting the worker's fitness for day work, prevent the worker from working at night. Age in itself does not appear to be a sufficiently valid reason to "automatically" trigger the exemption, but any conditions that expose the worker to a real risk of developing morbid conditions in association with age may be a sufficient condition for the worker's exemption from the same shifts.
The worker's ability to work night shifts is not affected by the medical condition.
There is no general rule, and suitability for night shifts is also determined by the physician in relation to the specific task of the worker.