Also, if you stop at the sleep stop, factor in additional time just to get off-route to the stop, prepare to sleep, and then after prepare to ride and to get going again. I reckon, from experience, there's a good 45 minutes lost in the faffage of a full sleep stop, on top of the time in bed — more, if you're trying to stick with a ride buddy. On my last 600 (The Flatlands solo route check) I managed an hour's snooze in McD's, amounting to a two-hour stop overall with breakfast.
I'll probably do my usual — an hour or a couple of halves as I go, with 20-minute sit-downs for the rest at the other controls. I'll be carrying a lightweight sleep mat for bus-shelter use should I need it, and we're forecast a really warm night, so no need for anything more than I'd be carrying anyway (rain jacket is usefully snug on such nights).
On a 6am start and based on my pace last weekend, I think I should be able to get quite a long way back to Bray before needing to stop, assuming I can bank some sleep over the next few nights (edit: not exactly bank, but just make sure I don't have much in the way of sleep debt, as Vorsprung said).