The proper celebration of the Sacraments often leaves me - and other priests - exhausted. I find that after two Masses in celebrated closely together leaves me quite worn out, and to celebrate a third Mass in short time is no easy feat.
Likewise, the celebration of Penance - when there are many penitents - also saps my energy. The exhaustion is heightened when Penance is celebrated after two Masses and a Baptism, as I did today.
The exhaustion following the celebration of the Sacraments is not so much a physical exhaustion, as it is a spiritual and emotional one; I am simply drained, feeling something like a zombie (presumably, anyway). Physically, I could keep going along easily enough, but having all but nothing left spiritually or emotionally there is little that can be done. Often times, all I want to do is sleep, but since my body is not tired I cannot.
It is not uncommon for people to express surprise when learning about this exhaustion, but if we consider what it is a priest does - or should do - it comes as no surprise.
When he celebrates the Sacraments, a priest allows himself to become the channel, if you will, for the power of God; he is the conduit of God's grace.
By the sacred powers entrusted to him at his ordination the priest invokes the Holy Spirit upon the things of this world. It is through the hands - and the words - of a priest that bread and wine become the Body and Blood of Christ. It is through the hands and words of a priest that sins are absolved and the sick are healed. Christ Jesus uses his priests to effect his presence in the world.
This is no simple matter. To invoke the power of God upon the things of this world a priest must lay himself aside and allow himself to be exhausted in the service of his Master. Physically, this takes very little effort, which is why so many are surpristed that a priest would be exhausted after celebrating Mass.
It is true, he has expended little physical effort, but the power and grace of God have flowed through him nonetheless and this is why he is exhausted.