"Sleepless" is, arguably, a mythology episode. "The Host" was the first episode in which we heard Mr. X, but we get to see him in "Sleepless". I don't like Mr. X as much as Deep Throat but he's still a fun character. Why I would argue that this is a mythology episode is because it basically introduces Krycek and Mr. X and also because it does deal with government experiments and a sort of conspiracy, albeit not one related to aliens. The introduction of two characters who would mostly feature in the mythology episodes make this a key episode for the storyline and hence not really a 'standalone' episode. It's also got a scene with the syndicate and CSM that is unlike basically anything you would see in a normal standalone episode, and leads directly into the following episodes, "Duane Barry" and "Ascension".
The concept behind "Sleepless" is interesting enough, and fairly well-written overall. It accomplishes what it sets out to do, basically. This is Howard Gordon's (later to become the showrunner for "24") first episode without writing partner Alex Gansa and it is a clear improvement over the Gordon/Gansa episodes from season one, although not better than "Miracle Man" which Gordon co-wrote with Chris Carter. Howard Gordon did write a few good episodes ("D.P.O." and "Grotesque" in season three are really quite good), but is still one of my least favorite X-Files writers. His stuff is sort of humorless and drab, and the dialogue, often the strong suit of many (most?) X-Files episodes tends to be rather cheesy and poorly-written in his episodes. In short it reminds me of "24" and I am NOT a fan of that show. Most of all these episodes lack the intelligence and wit, satire and irony that was a key presence through most of "The X-Files" even at its darkest and most straightforward.
"Sleepless" suffers from these same problems, but it's got a good plot, good direction by Rob Bowman, and features some important additions to the show story-wise, so in spite of the half-baked and drab script it is somewhat enjoyable overall.
6/10