Wired reports on why spending extra time sleeping in can actually make you feel more exhausted. This can help you explain to your patients why, when they think they’re getting more rest, they actually feel deprived.

Scientists believe that the pacemaker evolved to tell the cells in our bodies how to regulate their energy on a daily basis. When you sleep too much, you’re throwing off that biological clock, and it starts telling the cells a different story than what they’re actually experiencing, inducing a sense of fatigue. You might be crawling out of bed at 11am, but your cells started using their energy cycle at seven. This is similar to how jet lag works.