Morning. I lambed in December and once all had bonded, kicked out the back though All mums and babes were locked into a covered lean to at night. A vet once commented that my place was the livestock version of the Ritz! In other words i don't like livestock roughing it out over winter. I'm nuts! Anyhow, it all actually depends on the weather for your area and how well the shelter is constructed. If the lambs are weaned and doing well and can tolerate the bad weather during the day then in theory there shouldn't be any reason why they can't go out at night. We had sunny weather a couple of weeks ago so I kicked the cattle out, they tie in stalls, well the weather has changed and now they're looking at me. But, they do get the lean to at night! Sheep and fat lambs on another field!