WASHINGTON (AP) — U.S. health regulators on Friday approved the first drug to treat a sleep disorder that mainly afflicts the blind.

The Food and Drug Administration cleared Vanda Pharmaceuticals’ Hetlioz capsules for patients who have problems sleeping because they can’t detect light. The condition, called non-24-hour disorder, is estimated to affect up to 100,000 Americans, most of whom are totally blind. These people can find their sleep patterns reversed — sleeping during the day and being awake at night.