I first came across this movie on a late-night TV showing when i was around 11 years old, and for years some of the imagery stayed with me. William Hurt (in, i believe, his debut film performance - though it's a performance so nuanced you would scarcely credit that) became a favourite actor of mine due to his characterisation of brilliant but emotionally detached scientist Eddie Jessup, dealing with childhood trauma and lapsed religion by conducting dangerous experiments in sensory deprivation to bring about altered states of consciousness and explore his inner self.
Whereas most "man into monster" movies set up the lead character as a nice, stand-up guy to contrast with the beast he becomes (such as Larry Talbot in "The Wolf Man" and Henry Jekyll in almost any version of Jekyll & Hyde), Hurt gives us a complex, hard to like Eddie Jessup; a man who we can easily understand, but find it hard to empathise with. As Jessup grows more distant from his wife (the lovely Blair Brown, who i wish i'd seen in more things), and retreats into himself in a very literal fashion, things get very dangerous. A regression into a Neanderthal man leads to a zoo goat-based meal even more inadvisable than a drunken kebab, and Eddie from thereon in becomes more and more detached from humanity physically and psychically, as much as socially. The scenario in which we hear the primaeval bark from the 'sleep tank' is extraordinarilary powerful.
An ending scene still redolent today, possibly more recognisable because A-Ha stole it for the oft-played "Take On Me" video, caps a film that - unbelievably described by another reviewer as pointless and plot less - explores love, faith, life, death and what it means to be human. This is a film about everything, not nothing, and Eddie's journey from detached savant to primordial ape to weird energy thing, and finally to fully rounded human being is one that i highly recommend everyone goes along on.