|Year length (PST)||360 days|
|Day length (PST)||25 hours|
Source: Heart of Night, pg(s). 62
Oneirian's geography is similar to that of Nodethe: a typical habitable world with polar ice caps, oceans covering more than half of the surface, breathable atmosphere, abundant minerals and varied, edible or domesticable flora and fauna.
When a visitor to Nodethe sleeps, they disappear from it and travel to Oneirian with all the energy of a full sleep. When they fall asleep on Oneirian, they return to Nodethe. Natives of Oneirian similarly travel between both worlds, but only once or twice per month. If a native of Oneirian leaves Nodethe, they seem to disappear forever and never return to Oneirian, while if a visitor dies in Oneirian, they never return to Nodethe's reality.
The night sky on Oneirian is dreamy and starry, but does not have any stars. The space near Oneirian is empty, and planar travel from Oneirian is impossible. Common theories point to Oneirian being linked to the Dimension of Dreams, and ancient texts found in Oneirian's ruins mention locations on that plane like the Dreamlands and Leng.
Oneirian is home to numerous species inhabiting various cities that show signs of being millennia older than the first cities on Nodethe. An ancient cave painting in Oneirian suggests an incoming calamity that will merge Nodethe and Oneirian and exterminate all inhabitants of both worlds.