Temporary Hit Points

From HGWiki

Certain effects give a character temporary hit points. When the temporary hit points expire, the character's hit points drop to his current hit point total. If the character's hit points are below his current hit point total at that time, all the temporary hit points have already been lost and the character's hit point total does not drop further.

When temporary hit points are lost, they cannot be restored with healing as with real hit points.

Player characters should beware of resting when under the effect of a spell granting temporary hit points. If the temporary hit points are the difference between life and death the player character will die in his/her sleep!