So, what exactly happens when you hit the sack? According to the National Sleep Foundation, a night of healthy sleep should cycle through five sleep stages every 90 minutes or so. The first four stages make up our non-rapid eye movement (NREM) sleep, and the fifth stage is when rapid eye movement (REM) sleep occurs.
According to International Classifications of Sleep Disorders, shift work sleep disorder is a circadian rhythm sleep disorder due to unusual sleep patterns caused by changing work schedules. People with SWSD often characterized by insomnia, and excessive sleepiness during work hours.