Symptoms can vary from person to person, however daytime sleepiness is the most common. Sleep apnea doesn't typically fully wake a person up, just enough to interrupt the sleep cycle, affecting the quality of sleep; therefore causing sleepiness (as well as other possible symptoms) during the day.
Many of the symptoms that occur during sleep aren't noticed by the patient, but can be witnessed by another person. These can include loud snoring, unusual breathing noises, or even a noticeable lapse in breathing.