The ISS orbits every 90 minutes. Any non-trivial space-walk will be in darkness at least part of the time.

Depending upon the Beta Angle [1], the spacewalk could be in sunlight the entire time (though that's rare). It could be in darkness up to about 50% of the time.

[1] http://en.wikipedia.org/wiki/Beta_angle

For the ISS, an orbital day is about 90 minutes.

They are in low-Earth orbit. Each orbit only takes about 90 minutes, and half of each is daytime and half is night. Since space walks are typically several hours long, they are typically half in night and half in daylight.

the oribital period of the ISS is 92 minutes 50 seconds. Therefore they do not experience a 24 hour day cycle as we do here on Earth. So the term "nighttime" doesn't have the same meaning. However there are orbits that are more heliosynchronous than others providing more solar time. At this moment it seems that the ISS is in an pass that is more than 50% solar time as can be seen on sites such as this one: http://iss.astroviewer.net/.