The point of him only sleeping 4 hours, is surely a part of propaganda. Stalin lived slow life in opulent luxury, often sending off his generals during the war so he can finish his meal and have tea.
Just because they're capable of physically staying awake without much sleep, doesn't make them so superhuman as to remain unaffected by doing so.
The book is written by a Cambridge-trained historian who did a painstaking work of documentation, including reviewing previously inaccessible Russian archives and interviewing just about every possible Stalin relative and survivor at the time of writing.
Cambridge historian will he be, but he just reverberates the commodity propaganda. Stalin was closing on the godhood in just every written source.
I'll give you an analogy: if some super secret North Korean state archives claim that Kim had no anus, would the Cambridge trained historian claim the same just because of that? Common sense would be lacking there, as it is in popular Western depictions of Stalin. I would not be surprised that people were flattering the guy even in their private notes, for the fear of them being found not doing that enough.
Any well schooled Russian person has a more or less complete idea of who he was as a person. Now, would somebody be a Cambridge-trained historian or not, would he know more of how such type of people live without "feeling and breathing" their will and presence? I'll trust the popular hearsay more than work of acclaimed historian then.
Almost every Western account of Soviet despots made me laugh, trying to find some fine, intriguing, sophisticated persona, where there were none. Why are they trying so hard at that?
To me, it feels that Western historians often pull more effort to rehabilitate the soviets than the contemporary revisionist propaganda.