An estimated 15 million Americans routinely work night shifts or rotate in and out of overnight shift work, and, as anyone who's ever been stuck working when others are sleeping can tell you, the odd hours can take a toll.

A study published this month in the Journal of Clinical Sleep Medicine suggests that it's insomnia, rather than sleepiness, that has the largest impact on night shift worker productivity.

The study looked at several dozen permanent night workers, the majority of whom were diagnosed with "shift work disorder" - a condition whose symptoms include excessive sleepiness at times when the worker is supposed to be awake, alert and productive, as well as a lack of concentration and energy, irritability and chronic insomnia.