Dog barks at night in the hot weather.

Have a year old golden retriever, crate trained, water in the crate. Hot weather seemingly sends him a bit crazy at night. Will bark every 1/2 hours. Doesn't seem overheated in any way, no panting or distress and he's in the coolest bit of the house (about 18-20c at night). Last night I slept on the sofa near his crate and he was better, stirring every couple of hours but I settled him easily then got back to sleep.

Anyone else experienced this? Any tips? Or what exactly could be causing it? Ideally I'd like to sleep in my bed as I get a better sleep but it's not the end of the world if I have to sleep on the sofa this week. Could possibly have him upstairs with us but a) it's hotter up there and b) he's slept in his crate every night so don't want to affect his sleeping in the crate generally. It also happened last heatwave.