Besides the effect of the circadian rhythm, shift work greatly affects the rhythm of sleep itself.
The human sleep pattern is divided into two categories:
1. non rapid eye movement (NREM) sleep.
2. rapid eye movement (REM) sleep.
NREM and REM sleep combined, take around 90 minutes to complete and they
are repeated at the same frequency as the night wears on.
This can be seen in the diagram EEG Sleep Stages and Time of Night.
When a person significantly changes the time of their sleeping, the cycle of NREM/REM becomes de-synchronized. Some research has indicated that for this cycle to return to a fairly normal state, it can take between seven and fourteen days.
Therefore, a shift worker who changes their sleep pattern every four to seven days, has little chance of their NREM/REM sleep cycle returning to a quasi normal state.
Sleep is divided into four stages:
Stage 1 (NREM Sleep) - This is a transitional stage between waking and "light sleep." In this stage the body readies itself for sleep. This stage lasts between 10 and 15 minutes.
Stage 2 (NREM Sleep) - This is known as "clinical sleep." In this stage blood pressure drops and heart rate decreases. We are in this stage about 50 percent each night. Since daytime sleep is usually shorter, the night shift worker loses out on some of this stage 2 sleep.
Stage 3 and 4 (NREM Sleep) - These
stages are considered to be restorative sleep. In these stages the person
is in "deep sleep" and it is in these stages the body and mind repairs itself.
These stages are very important in maintaining a person's health.
During NREM sleep our minds are quiet and our bodily functions remain in their usual (although slower) states.
REM Sleep - "Rapid eye movement sleep" occurs usually between 70 and 90
minutes after the onset of sleep. On average, a person completes 5 cycles
each night with the first period lasting 15 to 20 minutes and the last 30
to 60 minutes. It is the shift worker that tends to miss out on these last
REM periods as they tend not to sleep as long during the day. During this
stage there is considerable mental activity. The heart rate speeds up and
blood pressure fluctuates. It is during this stage we dream. Everybody dreams,
but if you are not awakened in the REM stage, you tend to forget your
Another interesting fact about REM sleep is that the ability for the body to regulate temperature is lost and the body is paralyzed during REM sleep. This may be to prevent us from acting out our dreams.
A shift worker trying to sleep during the day typically sleeps 1.5 to
2 hours less than their night sleeping counterpart. They miss out on a
significant portion of REM sleep and the amount of stage 2 sleep is less.
This results in "fragmented sleep."
Less daytime sleep is a result of the altered sleep/wake cycle as well as dealing with external daytime noises such as phones, lawn mowers and dogs. It has been shown that noise, even if it does not wake one, has an effect on one's sleep cycles.
The night shift worker while on nights must deal with sleep deprivation
as well the effect of the trough of the circadian rhythm. As a result it
is difficult for a shift worker to keep from feeling tired.