Don't worry about it...
You are learning a complex new skill which is also a bit of an overall assault on the senses as well as being a mental challenge. The heightened state of arousal which is necessary to deal with this process is exhausting for many - I remember feeling exactly the same when I first began PPL training.
Human beings are generally very adaptable though, and after enough repeated exposure to this strange new environment you will probably learn to relax more as you become more confident and familiar with your flying, and will not feel so exhausted afterwards.