The clock watched constantly becomes part of what makes the wait tiring.

Measuring a wait adds to it. Every glance at the time restarts the count, confirms how little has passed, and turns a stretch you might have got through into a series of disappointments a minute apart. The tiredness afterwards comes partly from the waiting and substantially from the checking, which is effort spent on something that changes nothing.

Put the clock out of sight when you are waiting on something you cannot hurry. Set one alarm if you must know, then let the time go unwatched. The wait will be the same length and cost you considerably less.