Researchers tracking smartphone data say they recently made a disturbing discovery: For the first time since states began implementing stay-at-home orders in mid-March to limit the spread of the novel coronavirus, Americans are staying home less.

The nationwide shift during the week of April 13 was relatively slight. However, any loss of momentum, particularly when stay-in-place orders remain in effect across most of the country, has some public health experts worried about “quarantine fatigue.” Any increase in travel, they say, is premature when staying home remains the most effective way to limit the spread of the virus until widespread testing and contact tracing become available.

“We saw something we hoped wasn’t happening, but it’s there,” said Lei Zhang, lead researcher and director of the Maryland Transportation Institute at the University of Maryland. “It seems collectively we’re getting a little tired. It looks like people are loosening up on their own to travel more.”