Is sleep depravation a form of torture?

Yes. Sleep deprivation is very pyschologically damaging and causes a lot of stress and anxiety. It can even be fatal. Rats deprived of sleep die within a couple weeks. Even deprivation short of fatal can cause physical damage. Blood pressure increases, heart arrythmias can occur, and strokes or heart attacks can be brought on by sleep deprivation which can cause permanent neurologic and cardiac damage. Additionally, something similar to serotonin cascade syndrome can occur when enzymes needed to neutralize lipid peroxides produced in neurotransmitter metabolism are depleted by prolonged wakefulness, and this syndrome can directly kill neurons and cause brain damage. The long lasting effects of long term sleep deprivation caused by this brain damage can be permanent memory problems, emotional disorders like chronic depression or bipolar disorder, and locomotor ataxia, which causes tremors and tics. This sort of damage is also seen in methamphetamine addicts. Some of it is caused by the drug itself, but some is simply the result of being awake for days on end.

So, even though it doesn't involve application of physical force, sleep deprivation done with intent to weaken a person psychologically and get them to talk is most definitely torture. It's damaging to the vicitim's mental and physical health, both short term and long term, and can cause permanent disabilities or even death.

And, as with all forms of torture, the information you get from people is useless. If you apply enough stress, people will admit to anything, so you can't rely on the information. Consider Senator John McCain's experiences with torture, in which he admitted to being a war criminal, signed documents to that effect, and also provided the his Vietnamese captors with a list of other pilot's names, which was actually the roster for the Greenbay Packer's (IIRC) football team. So, while he did provide some real information, such as confirming that a nearby power plant was his bombing target, a lot of what he told them was simply nonsense or false confessions he offered up to try to make them stop, and his captors had no way of knowing what information was real and what was false. The result is that torture generally doesn't provide "actionable intelligence". In order to act on information, you must have the ability to know whether or not it's true, and confessions and information obtained by torture can only be determined to be true if there is independent confirmation through other intelligence sources, in which case you might as well just rely on the other sources of information and skip the torture altogether. So, ethical issues aside, there's no reason to commit torture. It doesn't really work.