I'm curious if it's possible for human life to survive in both of the following cases:

1. A fantasy world with only sunlight (no night).

2. A fantasy world with only night (no day).

Note: let's ignore astronomic concerns for now (things like tides or gravity if, say, the world was a planet that didn't spin) unless you strongly believe it's an issue no matter the details of the world structure.

If you think it's possible, explain how. If it involves ingenuity on the inhabitants' part, bonus points for "fitting it" into a fantasy realm (vs. science fiction). For example, your average fantasy peasant in the "darkness world" probably doesn't know about Vitamin D deficiency. However, it's possible that they learn (somehow) that "healthy" people eat fish.

I'm sure more clarification will be required on my part (and I'll edit my answer accordingly), but I'm trying to keep it pretty open for now.

I'm not requesting hard science--just plausible answers.

To start, a couple issues that I'm interested in ideas for:

Both worlds:

• Lack of a day/night cycle for humans and other creatures to sleep
• Extreme hot/cold temperatures (thanks, santyclause!)

Darkness world:

• Oxygen production (no photosynthesis)

EDITS

For clarification:

Both worlds:

• The worlds are linked: light world inhabitants who are in charge of governing / policing the dark world will travel between worlds often
• Portals: the only way that the worlds are connected are through "small" (a few meters in height and width) portals. All transportation between worlds must be done through one or more of these portals.
• Portal technology. The portals were not created by the light world inhabitants or the dark world inhabitants, and largely remain a mystery to both (other than that they know, generally, which portals lead to which places). Thus, they cannot be manipulated, but only used for their "intended" purpose. Another point that may be important to note: all portals are one-way. For example, if you take a portal from the light world to the dark, you'll need to know where to go to get to a portal that leads back to the light world. There are many more "intra-world" (only lead to other same-world locations) than there are "inter-world" portals.
• Light cannot pass through portals.
• Distance between worlds. They are very far apart, so much so that transportation outside of the portals is impossible for either civilization (even though the light world civilization is considered "advanced", they aren't advanced enough to transport materials / energy / etc. to the other world without the portals).

Dark world:

• One moon and stars (think nighttime on Earth), but many storms and clouds. I'm open to multiple moons if it would increase plausibility.

Light world:

• Inhabited by a relatively advanced civilization capable of building large structures and retrieving the dark world's resources (water, for example) via portals to sustain itself. NOTE: this does not mean that the civilization is very advanced technologically. Their scientific progress is still semi-medieval (a little more medical knowledge), though aided by magic.