There isn't a night-time warning. It is understood that when the map turns dark it would be either night fall or in a cave without lighting. As for deadly versions of enemies, it was only mentioned briefly in the tutorial about more dangerous enemies in the night. The tutorial only aims to provide information at the most fundamental level.
I greatly value your input for the tutorial and it is important to improving the game's basic guidance.
I'll review and improve the tutorial's critical information as much as I can for the full game.
For demo, unfortunately I would not have time to improve further on. It shall remain as is.
As always, please feel free to share any input or feedback that you feel that could enhance your enjoyment of the game.