Depending on how rigid you want Trance to be (aka how you fall on the "semi-conscious" spectrum).
Version 1: (more natural language)
Sleep Alternatives
Creatures that do not require sleep can perform 4 hours of light activity and gain 8 hours of sleep. Creatures with the Trance ability remain conscious, but must perform no activity.
Version 2 (more exact mechanics)
Creatures that do not require sleep can perform 4 hours of light activity and gain 8 hours of sleep. Creatures with the Trance ability are instead stunned for 4 hours or until they end the trance.
Stunned gives you teh majority of unconscious penalties but not all of them, so its a reasonable interpretation for "semi-conscious" if you want to be more exact and rigid. Or you can leave it more in natural language with the first version.