I read your question and I understand your concern. In sleep apnea ability to sleep itself is not affected, but because of the breathing difficulty disrupts the persons sleep. Most common cause is airway obstruction often in obese patients (not your case), with snoring during the night. Meanwhile in insomnia there is an inherent inability to fall asleep or to stay asleep usually because of neurotransmitter and hormonal disbalance with higher levels of neurotransmitters which regulate sleep. At times might be difficult to distinguish clinically insomnia due to inability to maintain sleep unless there are the elements of snoring and evidence of obstruction to direct towards sleep apnea. Also while sleep apnea is always related to daytime sleepiness that is not necessarily the case in insomnia, not in the diagnostic criteria (though it can be present). Insomnia is often found in concomitance with anxiety or mood disorders. I hope to have been of help.