Fever dreams...

[Mister Obvious]

When the hypothalamus works to increase body temperature to combat an infection, this can cause disruptions in sleep – leading to what we refer to as 'fever dreams'. The vivid fever dreams most likely occur during our REM cycle when our brain waves are most active.

That doesn't account for having fever dreams when there's no infection present, fever dreams solely due to heat.

https://amerisleep.com/blog/fever-dreams

The exact cause of fever dreams is unknown. Some researchers speculate that the 'overheated' brain affects cognitive processing and therefore makes the fever dreams more strange than typical dreams.

https://www.sleepfoundation.org/dreams/fever-dream

Okay, that definitely makes more sense.