The Joy Meter is part of the player Interface in We Happy Few. The meter tracks the character's Joy experimentation. If the Joy meter fills up, the character will overdose and suffer penalties to combat, crafting, and conformity until the overdose passes.
As the character takes more Joy, the will have memory loss issues as represented by a Memory Loss Counter. When the counter becomes full, the character will crash and suffer severe penalties. Use Joy only as needed for tactical reasons.