I've never checked outdoor lambers after dark, as the worst thing would be to disturb a ewe that's lambed one and started another, and have her not get back to her firstborn.
It seems to us that they lamb outdoors at dusk and at dawn or nearly, and not usually in between overnight, though they do sometimes lamb in full daylight. First check at early light usually finds any of the morning's early crop.
If we have one looks like lambing last thing, we may catch her and bring her in, just in case. If a lamb born at dusk in March up here doesn't suck, it'll be dead by morning.
It's different if they're indoors. We've put them in an unnatural situation, a more risky situation, so we owe them the extra care. More risky because of the possibility of infection - we iodine all navels indoors and not at all outside, for instance. More risky as indoors often means in closer quarters with other sheep, so increased risk of muddles, stealing, one sheep attacking another ewe's lambs, etc.
But yes we all need to recognise that an absolute top priority is a shepherd who's had enough sleep to think straight and has the energy to be able to look after his/her charges. So sometimes a calculated risk must be taken for the greater good.
If I had 20 ewes, tupped as one group over 17 days, indoors with plenty of room, then unless it looked as though anyone was likely to lamb in the night, I would probably leave them till first light. Even if one does lamb, it's likely to be just one, so hopefully no muddles.
However, if I had 20 indoors that I'd AI'd, so they're likely to lamb in a tight batch, then I'd be checking every couple of hours so as to get lambers off to a pen with their lambs before anyone tries to pinch 'em.
(And, sorry to disappoint, but I have had 5 lambs born to 2 ewes indoors at 2am, and a right royal muddle with the two mothers and one who hadn't started yet but claimed one of the new lambs...
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