The "heat of the moment" describes the immediacy and aliveness of being very involved in something. To be "in the heat of the moment" is to be in the midst of some activity and under the influence of that event. Example: "You never really know for sure what you will do in the heat of the moment." In the heat of the moment we sometimes do or say things which we regret later."He was so frustrated that in the heat of the moment he quit his job."  Sometimes people are "lost" in the heat of the moment, which usually means that they lose some aspect of self-control. "He was lost in the heat of the moment and tried to punch the police officer."