He was sort of embarrassed at having wasted so much time and mental energy on the project.

Until then, it was a waste of mental energy to try to second guess what would happen.

He sat for a few minutes after she'd left, unable to work up the mental or physical energy to do anything else.

Worry over money has sucked away the population's mental energy.

The project that was taking most of my time and mental energy wasn't going to get me lunch money.

"I just don't have the mental energy to get back to that 1-yard line again."

She was working on a retort, but it cost too much in terms of mental energy.

Yes, fish it was for the mental energy to face our new situation.

So she focused all her mental energy at the woman and thought: drop it!

But being all alone, talking to no one, so intense, you can drain your mental energy.