Yes it's healthier to sleep in the cold as long as the cold temperature is not freezing cold.
However for most people who suffer from Insomnia the actual cause is because they are too warm and the warm body has a much harder time at falling asleep and staying asleep.
Having your room a bit cooler can help you fall asleep faster and stay asleep.
I keep my room colder at night and find that I get much better sleep and wake up feeling refreshed.
If it's really chilly though then I cannot get much if any sleep.
But slightly chilly is okay for me.
Some people get better sleep when they are warm but some people get better sleep when it's colder.