A person may not be able to function when there is not enough sleep. Changes in mood, fatigue, and loss of concentration are common symptoms caused by lack of sleep. Lack of REM sleep, or quality dream-state sleep, may also lead to the same result. A person who does not have enough rest and sleep can experience headaches on a regular basis. On top of that, without regular REM sleep, a person starts to lose focus when awake. Sleep allows the body to recuperate and recover from the rigors of daily living. REM sleep will enable the mind to regenerate its contents. REM sleep can bring memories, concepts, and ideas to the fore, moving other memory back into long term storage.