I've been recording lambing times for the past few years (sad I know!) - black welsh, lambing outdoors.
Mine mostly either lamb between 4am and 8am, or between 8pm and 10pm.
I find my first timers (ewe-lambs) tend to lamb in the morning slot.
I think the more confident the ewes (i.e. least flighty, most friendly) the more likely it is that they will lamb outside those times, or if they know I'm around - it may be that they associate me with safety.
I had a ewe once who'd been in labour for a little while, but not long enough for me to be worried. Well, I went into the field to check on her (from a distance with the binoculars

) and she got up and walked straight over to me (they would normally stay put or walk in the opposite direction!) .... it took me a while to cotton on but it turned out she was asking for help, the lamb was backwards and she obviously knew she wasn't going to get it out by herself! I never underestimate them now

.