Use when describing physical or mental tiredness after sustained effort or activity. She had been studying all week for her exams and finally ran out of steam.
Pro Tip 2/3
Informal Expression
Best suited for casual conversation rather than formal settings. I wanted to finish painting the house today, but I've run out of steam.
Pro Tip 3/3
Waning Interest
Can imply lost interest due to fatigue, not only physical exhaustion. I was excited about learning to play the guitar, but I've run out of steam.