18-20 deg C for short duration exposure is still bearable but if 18-20 Deg C > 1h, it is freaking freezing cold.

Advisable to wear a Long Johns internally before putting up any other long pant or long sleeve shirt / jacket.

If day time 18-20 Deg, expect night time to fall by 6 to 7 deg C = 11~13 deg C range.

If your hotel got heater, it should not be an issue. After this, you will probably understand why 4 seasons climate country's ppl love the heater.