Two suns shone in the sky above Metru Nui. These suns rose and set every day.
Although much of Onu-Metru was on the surface, Onu-Matoran preferred to work in the underground levels of the Archives, because the brightness of the twin suns hurt their sensitive eyes.
"Dawn was breaking over Po-Metru. As the twin suns shed their light on the canyons, Matau, Nokama, and Vakama lay flat on a ridge, surveying the territory."
"Turaga, you said I must 'stop the darkness.' Bust sunsfall isn't for —"
As of story year 2008, the sunholes shone with muted light.
After Teridax's takeover, the twin suns in Metru Nui turned red because of Teridax's eyes.
The twin suns over Metru Nui represented the eyes of Mata Nui. The day-night cycle of Metru Nui was tied to Mata Nui's sleep cycle; Mata Nui was asleep when the suns over Metru Nui were down.
Sunlight came through holes in the "sky" over Metru Nui.