You cannot burn the candle at both ends, so with the best will in the world it's not possible for one person with your responsibilities to be able to check them regularly throughout the night as well as the day. And the more sleep you miss, the more likely you are not to notice any signs of trouble.
Your ewes that have lambed before will be the least likely to have any problems. So your shearlings, especially as they are expecting singles are the ones to watch most carefully.
Are your sheep outside? If so, a classic sign of impending lambing is when a ewe takes herself off to a far corner to be on her own. As you've noticed - they don't always go off their food or give any obvious signs until you see the classic string of mucus, or water bag emerging. So, if you check last thing at night and first thing in the morning, and they all seem to acting normally, then hopefully nothing untoward will happen overnight.