One aspect of Lord of the Flies is that while the boys experience extreme physical deprivation in the form of dehydration and starvation, they also experience all of the mental and emotional strain that comes from being stranded on an uninhabited island.  This experience would be hard enough for an adult, but for a child, being away from the protection of the adult world, is even more of a strain; with this thought in mind, the reader can easily comprehend why several of the boys seem to experience a mental breakdown. 

Simon, for example, already comes to the island with some sort of debilitating illness that causes fainting 'spells; his condition worsens under the oppressive heat of the jungle, and in chapter eight, "Gift for the Darkness," the reader sees Simon have a hallucination in which he converses with a pig's head, blacks out, and wakes up with a nosebleed in the next chapter.  This episode would definitely be classified as a mental  health crisis.

Ralph, too, experiences 'mental crises' in the form of regression and memory lapses.  In chapter seven, Ralph notices that he has started biting his nails again, "though he could not remember wehn he restarted this habit not any time when he indulged in it" (109).  Ralph's nail chewing signifies regression to earlier habits--comfort habits--from when he was a smaller child.  Ralph remarks upon noticing the nails that he will "be sucking [his] thumb next" (109).  He turns to these habits to diffuse the enormous amount of stress he feels, but then cannot remember when he actually chewed on his nails.  Golding includes other details to suggest Ralph's memory loss in conversations with other boys, like when in Chapter Eleven, when the boys prepare to visit Castle Rock.  Ralph can hardly remember about keeping the signal fire lit. When the other boys concernedly look at Ralph, he protests:

"'I hadn't,' said Ralph loudly.  "I knew it all the time.  I hadn't forgotten.'
Piggy nodded propitiatingly.
'You're chief, Ralph. You remember everything'" (173). 

Although the other boys assure Ralph that he remembered, it is evident that they recognize that Ralph's memory and control is slipping away.  The narrator observes that "the twins were examining Ralph curiously, as though they were seeing him for the first time" (173).  Clearly the emotional and physical strain of the island has impacted Ralph's mental health, resulting in memory loss and mental fatigue.