In short, insomnia. There are other sleep terms of interest (Sleepnet.com) but insomnia is about inability or interruption/wakefulness of a sleep pattern. Or, simply Sleep Interruption.
There are more specific terms depending on cause and experience.
Part of the problem with assigning a legitimate opposite of nap is that, in general, naps are scheduled and intentionally initiated (conditions like narcolepsy and drug effects aside). Interruptions in REM sleep are generally not consciously directed wake-up.
Certain conditioning of sleep patterns may not be legitimate insomnia, but instead lead to a normal[ized] wake-cycle that embeds awake time between naps (See: Polyphasic Sleep).
I am not a doctor. This is not intended as medical advice.