You can cover your under-eye circles with a dab of makeup.

You can drown your yawns in a giant mug of coffee.

But if you really want to mask your sleepiness, you may need to bite your tongue … because your voice may just give you away.

According to a recent study, the signs of fatigue show up in your voice in a few different ways. First, your speech slooows down. Australian scientists have actually pinpointed when this vocal deceleration starts to occur, too. They examined a group of eighteen people, keeping them awake for twenty-four hours to record samples of their voices at different intervals.

This first sign of sluggish speech occurred about sixteen hours into the experiment, just after midnight. But the groups’ speech was at is slowest right around dawn, when the effects of a full day awake were really taking their toll.

But slowness wasn’t the only characteristic of tired talking. The pitch and tone of your voice changes, too. The scientists found that their study subjects’ voices varied more in pitch as they got tired, meaning their speech wavered between high and low.

Experts say understanding how to measure fatigue through a person’s voice could help people avoid accidents. A National Transportation Safety Board study has linked between thirty and forty percent of fatal heavy-truck accidents to fatigue. And according to the A-A-A Foundation for Traffic Safety, about forty-one percent of people say they have fallen asleep at the wheel at some point.

Recognizing tired talk could also help different industries known for their long hours, like law enforcement and health care, get a better handle on just how tired their folks are and how to better manage it.

So, if your voice is starting to give you away, maybe it’s time you made a date with a fluffy, white pillow and called it a day.