The summer is a time of year that is used to gain strength and endurance.  Training loads can be increased for some athletes.  Please remember that recovery is an integral part of achieving your goals.  Hard work plus good nutrition and sleep must also be accompanied by relative rest or active recovery.  A simple method I use, is to check the resting heart rate first thing in the morning.  If it is increasing, you may not have recovered enough and should consider backing down.  Another clue is muscle soreness still present the following day as this can indicate you are not fully recovered.