Sleep deprivation can have serious effects on your health in the form of physical and mental impairments. Inadequate rest impairs our ability to think, to handle stress, to maintain a healthy immune system and to moderate our emotions. In fact, sleep is so important to our overall health that total sleep deprivation has been proven to be fatal: lab rats denied the chance to rest die within two to three weeks.

Impair immune system. If you know anyone with an impaired immune system you might understand. The word fatal surely gets you attention.

As for the definition of torture. This is what the US has agreed to:

any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him, or a third person, information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in, or incidental to, lawful sanctions.