Energy is defined as: en·er·gy (noun):  the strength and vitality required for sustained physical or mental activity. We rely on energy to get through the day, the week, the year.  We know that losing out on sleep can leave you drained, but sleep deprivation is only one of a long list of possible reasons behind feeling exhausted. We rely on energy to get through the day, the week, the year. We know that losing out on sleep can leave us feeling drained, but sleep deprivation is only one of a long list of possible reasons behind feeling exhausted.