Research shows that the most common time for energy slumps is 2.16pm. This is when many people hit a brick wall– and wish they could hit a pillow.

Does "hit a brick wall" mean "to fall asleep"?

No, it means to suddenly feel completely exhausted.

The idiom comes from marathon joggers who "hit the wall" at about 20 miles into the race.

So as you can see, it’s no accident that many marathon runners hit the wall at around 20 miles. That’s when they run out of glycogen and they become hypoglycemic.

HUBLOTIs "hit a brick wall" a British idiom?
The idiom in BrE is 'be/come up against a brick wall", meaning 'be unable to make any progress because there is a difficulty that stops you'. (Oxford Advanced Learner's Dictionary)
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Is "hit a brick wall" a British idiom?
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