On break

From DwarfFortressWiki

Yes, dwarves do have days off; all dwarfs will go on break for some time sooner or later. There is nothing you can do to stop it happening, but it's less annoying than Parties at least.

[edit] How to stop breaks

They will take more breaks if they are thirsty or have no access to alcohol. Give them lots of booze to keep them active.

You can cancel dwarves breaks by drafting them into the military and then immediately releasing them. This causes an unhappy thought if the dwarf has no military skills at novice or better. Some players consider this an exploit, while others think it is balanced by the unhappy thought.