When you understand how the body’s circadian systems work, it’s easy to see how keeping an irregular schedule will affect this internal timekeeper.
Shift work creates a misalignment between your internal clock and the outside world. When this happens, your body’s timekeeper sends you signals that conflict with the activities you’re trying to do. This may cause your body to secrete drowsy-making chemicals when you’re on the job, or alert you to be awake and eat when you’re trying to sleep.
The internal clock is somewhat flexible, so it can learn a new schedule and eventually adapt. For example, if you fly across time zones, your internal clock will initially be out of sync with the external environment, but if you wake up, eat, and most importantly, expose yourself to light on the schedule of your new time zone, your internal clock eventually adjust.
If your shift work schedule rotates, or you sleep with a different pattern on your days off, though, it’s like having continual jet lag —the circadian system never gets the chance to fully catch up.
Even if your shift work schedule does not rotate, if it requires sleeping during daylight hours that can be a challenge. Your internal clock, re-enforced by light and other external signals, naturally sends you alerting signals during the day, so even though you may fall asleep quickly after a night shift, it can be hard to sleep continuously in the day for the full 7-8 hours your body needs, because your body’s clock is cueing you to be awake. It’s hard to stay alert and productive at night for the opposite reason—the internal clock naturally causes your alertness to dip, so the pull to sleep can be powerful.