The reason it is a problem is because it SlumberPM effects the body' s circadian rhythm or natural sleep cycle. The circadian rhythm is an internal biological body clock that make you feel sleepy or alert at regular times of the day and is set by exposure to sunlight. The worst effected are those who are on rotating shift cycles.
Well symptoms can include, insomnia, constant tiredness or excessive sleeping, a distinct lack of energy or feeling fatigued, difficulty in making decisions, difficulty in concentrating and just a general lack of "get up and go" spirit. If you feel any or a number of these symptoms then chances are that you might be a sufferer. If you have to work shifts then it is important to try to settle into some sort of a routine ie normal bedtime hours and normal wake up hours that are applicable to your shift.
This way you can restore some sort of balance to your already imbalanced biological clock. If you work extra hours one day and not the next, then this can do more harm than good. However if you do have to do overtime, try to do it at the end of your working week as this will give your body time to adjust.