Does Face ID work in the dark?

It does. The phone screen will light up, and illuminate your face.

click here ➜ About Face ID advanced technology - Apple Support

Face ID is designed to work with hats, scarves, glasses, contact lenses,

and many sunglasses. Furthermore, it's designed to work indoors,

outdoors, and even in total darkness.

Curiousmonkey-1234 wrote:

Does Face ID work in the dark? Fingerprint ID works in the dark because you are in contact with your phone or computer. So I was wondering if you where left in the dark because your electric went off at night or lost in a cave and you needed to use your phone would the Face ID still recognize your face so you could easily unlock your phone?

Remember, you can always unlock your phone with the passcode.