John Axelsson has in his thesis at the Department of Psychology at Stockholm University and at the Institute of Psychosocial Medicine evaluated the effects of compressed working hours.
The main aims were to evaluate the effects of long working hours (in the form of 12h shifts) and short recovery periods between shifts (shorter than 16hours). Another aim was to evaluate possible mechanisms that could suggest why some individuals develop problems with shift work while others do not.
There was no convincing evidence for extended shifts – 12h shifts – inducing more problems with sleep and sleepiness than 8h shifts. The more days off seemed to improve the possibilities to recover properly and counteract possible negative effects of extended shifts, provided the number of consecutive shifts was limited. On the other hand, short recovery times (only 8h) between shifts seriously shortened sleep duration and increased sleepiness. 12h of recovery (between two consecutive 12h shifts) was judged as having no or limited effects on acute measures.
“Vulnerable” shift workers had more problems with sleep and fatigue than tolerant workers. Their problems were worse during work periods and disappeared during days off. Possible mechanisms for their problems were a larger sleep need and lower testosterone levels (amongst men), that also reflected their problems to get sufficient recovery during work periods, but allowed sufficient recovery during days-off when sleep was uninterrupted by work. Another countermeasure against disturbed recovery was a short nap, which restored alertness and performance back to normal.
It is concluded that long shifts (up to 12h) may be acceptable, whereas short recovery time (8h or less) is not. Most of the problems with short recovery periods were related to short sleep and sleepiness, and there is, clearly, a subgroup of workers that suffer more from this than others. It is argued that insufficient sleep and low testosterone levels (in males) might be key factors for developing shift work intolerance, mainly by reducing the capacity to recover from shift work.