They were all the same, days and nights; segments of time divided by a bell, different only in the external light.

"Although its external lights were on, nothing could be seen inside the cabin" the accident report stated.

Her path was illuminated by an array of ten external lights.

But biological time and external light aren't completely independent.

It's got an external light above the screen to illuminate the keyboard.

Since the city was blacked out for the night, the external lights were off.

At the last minute I remembered to turn off the external lights.

The rack of shelves is covered in black cloth to block out external light.

These rhythms are also strongly associated with external light and dark cues.

They are particularly useful for external lights, and can pay back their costs within a year.