Dark adaption 1

Be prepared to let your eyes adjust to the darkness. It takes about half an hour for the human eye to become totally darkadapted. If you look into headlights, or a bright flashlight, even for a second, be ready to start that half hour over again.

Stargazing Basics
Getting Started in Recreational Astronomy
ISBN-13 978-0-521-72859-1
- PAUL E. KINZER