Wiktionary
lightening up
vb. (present participle of lighten up English)
Usage examples of "lightening up".
When we got to the top, the sun was higher in the sky and the day was lightening up.
The rain had slacked off, and the sky was lightening up in the west.
SMS can use a little lightening up, and I think the Mischief Knights are just the guys to do it.
The sky was lightening up now, too much light for them to use their night-vision goggles, but not enough to see very well.