First be sure there is not a medical issue such as a blockage. A pup or dog that will not settle down at night could be a sign of a blockage or other issue.
Labrador pups do not like it hot. When my litters won’t settle down at nap time and are crying here is what I do:
Take small towels (dish size) and run under cold water. Wring out and store in the fridge.
Freeze plastic bottles of water and store in freezer.
Lay towels out in the whelping box or crate and the pups will flop down on their bellies and go right to sleep.
The water bottles work well for pups to lay on or sleep up against to also help them stay cool
Here is a pick : Scroll down to Jazz/Roger litter