Humans are
by nature diurnal (day orientated) as opposed to
nocturnal (night orientated) beings, meaning that our
physiological functions are geared towards day time
activity and night time rest.
Research has shown in fact that our physiological
functions fluctuate in cycles, or rhythms, which are
regulated by an internal biological clock
in the brain. These rhythms range from minutes, for
example in the stages of sleep, to days, for example
in the female menstruation cycle.
Studies have shown that the main physiological
functions such as core body temperature, hormone
production, heart rate, blood pressure, gastric
activity, and the sleep/wake cycle,
all have cycles or rhythms of approximately 25 hours.
These cycles are known as circadian rhythms (from the
Latin 'circa' = about, and 'dia' = a day).
Normally our circadian rhythms are synchronized
to one another by the internal biological clock, and entrained
(daily reset) to the 24 hour day/night cycle by external
time cues, namely the variation in sunlight
and the increase in environmental and family activity
around us.
A typical cycle of core body temperature, for example, normally has
two peaks (around 1130hrs and 1900hrs) and two troughs
(around 0400hrs and 1400hrs) - see attached
Core Body Temperature Graph.
When we work rotating shifts, we have to adjust
our natural sleep/wake cycle to suit whichever shift
we are on. However, our other circadian rhythms do
not adjust instantaneously - in fact different
rhythms change at different rates - and typically
take a week or more to adjust to the new sleep/ wake
cycle. A useful analogy is that of an orchestra where
initially the different sections are playing in
harmony under the one conductor. If another conductor
appears and starts a different beat there will be
discord as the different sections change at their own
rate to the new beat.
Rotating onto night shift causes the greatest
disruption to our circadian rhythms as we attempt to
remain active and alert during the night when our
circadian rhythms are falling to their lowest and try
to sleep during the day when our circadian rhythms
are rising to their peaks. When this happens we can
experience sleep loss and gastrointestinal problems.
Loss of sleep leads to, fatigue, irritability and
lethargy and a decline in performance such as poorer
concentration, alertness and decision making. This in
turn increases the risk of making errors which may
effect not only our own safety but also that of our
colleagues.
How quickly our circadian rhythms adjust to a new
sleep/wake cycle depends on internal factors such as
the age, gender, physical health and chronotype (
see 'Larks and Owls' ) of the individual, and
external factors such as the shift pattern and work
load, and family and social circumstances.
If we rotate onto another shift before our
circadian rhythms have had time to adjust to one
sleep/wake cycle, then our circadian rhythms may
become in a perpetual state of internal
desynchronisation, where they are never
fully synchronised to one another.
Although the effects of shift work on our
circadian rhythms cannot be eliminated completely,
there are steps that can be taken at both the
organisational level and individual level to minimize
these effects.