People think they're showing loyalty and effort by ignoring their biological need. The most poignant example, for me, is the Challenger tragedy. The shuttle people preparing for that launch were pulling double shifts. The Challenger commission report indicated a possible relationship between their lack of sleep and the impaired judgment that led to disaster. Sleep may play a major role in, among other things, regulating mood and emotion.
Most studies done on sleep deprivation look at performance, ability to think, reaction time.
Sleep deprivation is not the root cause of these problems, but the extent to which it contributes may be underestimated. The need for sleep varies like most biological functions, but you can fit the differences among virtually all adults in about an hour's span. It's likely that people who, in casual conversation, say they're fine on four or five hours a night have been acculturated to not value sleep or what sleep does for them. Or they're supplementing their waking activity with stimulants such as caffeine and nicotine. People are not good at perceiving their own sleep debt.