Tired is normal. Numbness, not so much. I would stop playing and give my hands a rest before they got numb. Check your posture and hand positions to see if that might be causing the numbness. Make sure your hands are warm. I like to run them under warm/hot water for a bit and do light stretching both before and after. Other than that I just think it's repetition i.e. exercise that will build up your strength.