I've read a lot of fic about Aziraphale learning to sleep, or Crowley convincing him to try sleeping when Aziraphale has never seen the point, but for me this piece plays off a very poignant idea that--now that I'm seeing it--seems very obvious, especially for Show Omens, but that I've never once seen explored: maybe Aziraphale doesn't sleep because it's not safe.
We know Hell "never check[s] up" on Crowley and is severely understaffed, which explains how Crowley has been able to develop a sleeping habit: he's not any less safe from Hell asleep than he is awake. And we know demons can't enter the bookshop without invitation.
In fic and meta this is presumed to be due to "wards" Aziraphale has placed over the property, i.e., continuously acting miracles; but we see in canon that miracles require an occult or ethereal will to occur: as far as we can tell from what we're shown in the series, they're discrete events consciously willed into being by their directors. Aziraphale pulls power "down" from Heaven and Crowley pulls power "up" from Hell (and before the creation of the cosmos, the Starmaker pulls power from everywhere around themself) to make them happen.
There's no indication either way of whether a miracle like a ward is discrete or ongoing; if it's continuous, like an electrical field, that may require the continuous will and conduit of the miracle-doer. If that's the case, what happens to the ward if the miracle-doer falls asleep?
Even if that's not a concern, we do know for sure that before Aziraphale rebels against Heaven and establishes the bookshop as an "independent Heavenly embassy," Gabriel and Sandalphon can and do barge right in anytime they feel the need to put the frighteners on Heaven's misfit angel.
And then there's the symbolic meaning of the bookshop's oculus--not just the omniscience of God, but an eye always staring down from Heaven even into Azirphale's private space, the panopticon's someone could be watching at any given time.
And there's that Heaven-white Roman head, occupying the centre of so many of the S2 bookshop scenes--again, Heaven (specifically the Metatron, as he's the other floating head) spying on their own. (The head is also a reference to iconic British tv series The Prisoner, in which Cold War superpowers use busts and most notably a Roman bust as a/v surveillance tools against their own agents.)
Gabriel's already got a Thing against Aziraphale eating, which Aziraphale has to excuse as blending in and doing as humans do and defend by pointing out Gabriel's own love of human tailoring. What would the Supreme Archangel and his enforcer do if they found their Earthside employee sleeping on the clock?
Season 2 of Show Omens establishes that, although Crowley and Aziraphale now associate openly, they still understand themselves to be in danger from both their respective erstwhile employers--enough danger that Aziraphale moves to protect Crowley from demonic intrusion by assuming partial ownership of the Bentley and Crowley prefers to let the bookshop remain "technically" Aziraphale's even though Aziraphale gives him to understand that it belongs in fact and use (just not in contract) to both of them. The places a human would consider "home" or "safe" are neither for Aziraphale and Crowley, and they both know that. So even after Aziraphale stops working for Heaven, it may still not be safe for him to sleep.
All of this crystallized for me because of this piece. Because look at Crowley's face. You can see from @eklmrlos' previous pieces that they're very fucking good at putting exactly the facial expression on Crowley they want us to see, and that, along with the picture's caption specifying this is at the South Downs cottage, makes me wonder if maybe this is a depiction one of the first times Crowley has ever seen Aziraphale asleep.
Whether it is or not, it's unquestionably about the fact that Aziraphale is sleeping: he's what's presented to the viewer (and what Crowley's looking at), and the first two sketch panels show us that Crowley has touched him and even kissed him on the ear and Aziraphale hasn't woken up. So the significance of the picture lies in the fact that Aziraphale is asleep and doesn't wake up to mild interference.
I would argue that this is specifically about the vulnerability of sleep, too. The two sketch panels illustrate the need to sleep with someone trustworthy because you don't have control of what they do to you while you're sleeping--and if they do it softly enough you may not ever even know about it. @eklmrlos also chooses to depict Aziraphale with his belly exposed to the viewer, an expression of vulnerability that runs so deep humans share it with thousands of other species. You don't even have to be a mammal to understand what an exposed belly means.
And Aziraphale's belly in particular is shown here as softer than the appearance currently demanded by our diseased culture; it's also a part of himself that Good Omens fans are aware he covers with at least three layers of clothing, and these specific notes make the vulnerability of its exposure--and the significance of the moment being shown--hit even harder. (It tickles my nerdheart so much when artists achieve a layer of meaning with their art that only exists because it's fanwork.)
But it's Crowley's face that drives home that the safety of this moment is special. Because that's the expression you wear when you're in a museum looking at the painting you've always wanted to see in person but never thought you'd get to, or a zookeeper lets you feed a tortoise bigger than you are, or you look up at the sky at night: it's a face so intent on drinking in what it's seeing that it almost, but not quite, forgets to show its owner's happiness.
