In a culture that prides itself on having an unhealthy relationship with work, getting enough sleep can get pushed way down to the bottom of our priority list. But the truth is that overdoing it makes us worse at our jobs—and can increase our odds of getting sick. A new paper published in the journal Sleep explains how fatigue can damage our immune systems.

Scientists recruited 11 pairs of adult identical twins. In each pair, one twin reported regularly sleeping well, while the other had trouble. All the participants were given wrist-mounted activity monitors, which they slipped on every night at bedtime for two weeks. The researchers also took blood samples from everyone on the last day of the study and sequenced their RNA, looking for differences in gene expression.

On average, the well-rested twins were getting about an hour more sleep per night than their unfortunate siblings. And that hour of sleep was clearly reflected in the poor sleepers’ genes. Their RNA showed decreased immune system activity in some areas and increased inflammation in others—making them more vulnerable to both disease-carrying germs and the illnesses and symptoms caused by inflammation. In short, the sleep deprived twins were far more likely to feel lousy.

Lead author Nathaniel Watson is co-director of the University of Washington Medicine Sleep Center. "The results are consistent with studies that show when sleep deprived people are given a vaccine, there is a lower antibody response,” he said in a statement, “and if you expose sleep deprived people to a rhinovirus they are more likely to get the virus. This study provides further evidence of sleep to overall health and well-being particularly to immune health.”

Alright, so we should be getting more and better sleep. But what does that look like? One recent article in the journal Sleep Health listed four criteria:

1. You take half an hour or less to fall asleep.
2. You wake up no more than once per night.
3. If you do wake up in the middle of the night, you fall back asleep within 20 minutes.
4. You’re asleep for at least 85 percent of the time you spend in bed.

If these statements seem absurd to you, it might be time to talk to your doctor.