Some warmer places drop amazingly at night, more arid regions have reputation for this. Less in tropical. Also even if 18 overnight doesn't seem cold when you're used to 40 by day, anything under 20 seems cold.

Im Brisbane and one time my partner was skyping with her Nordic family. I was getting the fireplace going and they asked the temperature, I cant remember exactly but it was about 20 and any the look on their faces was priceless.