Maybe most creatures on such planets would evolve towards unihemispheric sleep, as some Earth species did (eg. dolphins).
Unihemispheric slow-wave sleep (USWS) is sleep with one half of the brain while the other half remains alert.
https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep
Light/dark is only one factor in sleep, there are others.
It's also hard to know of the infinitely many evolutionary twists and turns lifeforms in the Universe may take, how many of those pathways involve something like sleep - especially on planets without a diurnal cycle.
Anything and everything is a wild guess based on a sample of one where extraterrestrial life is concerned.
Although they may never get sunlight, I suspect the view in their sky would be spectacular indeed (like ours when away from cities and light pollution, but even better.)
Maybe all that unfiltered starlight would be enough to coat the planet in a unique twilight all around, or maybe life there would specialize in other senses besides sight.
The planet may generate sufficient heat from its core and most of the life there might live underground, or there might be some other heat-providing geochemical processes on the surface.
[0] https://en.wikipedia.org/wiki/Rogue_planet
[1] http://www.bing.com/images/search?q=Milky+Way+from+Earth
The thermal vents from volcanic activity seem to be reasonably well insulated from cyclic activities at the surface. They would likely only see variation from cycles with much longer scales, such as the 26 ky axial precession cycle, the 41 ky axial tilt cycle, the 100 ky orbital eccentricity cycle, the 112 ky apsidal precession cycle, the 300-500 My tectonic supercontinent cycle, etc--the ones that can change climate and geology, rather than just weather.
The vent dwellers might get an inkling if an ice age has been going on the surface for a while, or if runaway greenhouse effect is boiling off the ocean surface.