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SLEEP AND HEALTH 37
night spent awake, will be reduced to 2 hours if a normal night’s sleep precedes it
and to 3.5 hours if a 2-hour nap is allowed (Åkerstedt and Gillberg, 1986). Thus,
the time of sleep termination depends on the balance between the circadian and
homeostatic influences. The circadian homeostatic regulation of sleep has also been
demonstrated in great detail in studies of forced or spontaneous desynchronization
under conditions of temporal isolation and ad lib sleeping hours (Czeisler et al.,
1980; Dijk and Czeisler, 1995).
2.3.4.1 Alertness, performance and safety
Night-oriented shift workers complain as much of fatigue and sleepiness as they do
about disturbed sleep (Åkerstedt, 1988). The sleepiness is particularly severe on the
night shift, hardly appears at all on the afternoon shift and is intermediate on the morning
shift. The maximum is reached towards the early morning (05.00–07.00).
Frequently, incidents of falling asleep occur during the night shift (Prokop and Prokop,
1955; Kogi and Ohta, 1975; Coleman and Dement, 1986). At least two thirds of the
respondents report that they have experienced involuntary sleep during night work.
Ambulatory EEG recordings verify that incidents of actual sleep occur during night
work in, for example, process operators (Torsvall et al., 1989). Other groups, such
as train drivers or truck drivers show clear signs of incidents of falling asleep while
driving at night (Caille and Bassano, 1977; Torsvall and Åkerstedt, 1987; Kecklund
and Åkerstedt, 1993). This occurs towards the second half of the night and appears
as repeated bursts of alpha and theta EEG activity, together with closed eyes and
slow undulating eye movements. As a rule the bursts are short (1–15 seconds) but
frequent, and seem to reflect lapses in the effort to fend off sleep. Approximately a
quarter of the subjects recorded show the EEG/EOG patterns of fighting with sleep.
This is clearly a larger proportion than what is found in the subjective reports of
episodes of falling asleep.
As may be expected, sleepiness on the night shift is reflected in performance. One of
the classics in this area is the study by Bjerner, Holm and Swensson (1955) who
showed that errors in meter readings over a period of 20 years in a gas works had a
pronounced peak on the night shift. There was also a secondary peak during the
afternoons. Similarly, Brown (1949) demonstrated that telephone operators connected
calls considerably slower at night. Hildebrandt, Rohmert and Rutenfranz (1974)
found that train drivers failed to operate their alerting safety device more often at
night than during the day. Most other studies of performance have used laboratory
type tests and demonstrated, for example, reduced reaction time or poorer mental
arithmetic on the night shift (Tepas et al., 1981; Tilley et al., 1982). Flight simulation
studies have furthermore shown that the ability to “fly” a simulator (Klein,
Bruner and Holtman, 1970), or to carry out a performance test (Dawson and Reid,
1997) at night may decrease to a level corresponding to that after moderate alcohol
consumption (>0.05% blood alcohol) Interestingly, Wilkinson et al. (1989) demonstrated
that reaction time performance on the night shift (nurses) was better in permanent
than rotating shift workers.
If sleepiness is severe enough, interaction with the environment will cease and if this
coincides with a critical need for action an accident may ensue. Such potential performance
lapses due to night work sleepiness were seen in several of the train drivers
discussed earlier (Torsvall and Åkerstedt, 1987). The transport area is where
most of the available accident data on night shift sleepiness has been obtained
(Lauber and Kayten, 1988). Thus, Harris (1977) and Hamelin (1987) demonstrated
that single vehicle accidents have by far the greatest probability of occurring at night.
NIGHT NOISE GUIDELINES FOR EUROPE