its because u have buylt up too mech qdrenaline!"1 u need to relax your self totally-when u get home after swimming try and meditate-there is lots of info on the net on how to do this, sit in a dark or darkened room (your bedroom maybe) clear your head of all thoughts and push amy thoughts coming in away- and and just concentrate on the 'nothingness' the here and now, and this should calm u rite down-do it 4 5-10 mins, or as long as you want to. this is also great in helping people who have anxiety, depression or panic attacks, and good 4 sports people who want to wind down. After, turn the lights off and try to sleep. Make a routine after u come back, have a warm relaxing bath with candles (NOT the bathroom light) or try the air wick flameless candle if that suits u better, have a milky drink-absolutly no tea or coffee, and dont drink red bull or lucozade during ur swimmin sessions, then meditate, then slowly try and fall asleep, breathing slowly and normally and u shouls start to fall asleep x