Research shows that untreated sleep apnea is correlated with:
Excessive daytime sleepiness
High blood pressure
Double the risk of stroke in men 
Increased risk of motor vehicle accidents
Increased risk of fatal and non-fatal cardiac events 
Greater likelihood of sudden death during sleep 
Increased risk of diabetes 
Increased risk of death from any cause 
Other adverse health effects of sleep apnea include:
Sleep apnea and obesity often occur together. Obesity contributes to sleep apnea and sleep apnea increases the risk for weight gain and makes weight loss more difficult.
Sleep apnea can be an underlying cause of chronic headaches, especially those occurring in the morning.
 Sleep Heart Health Study, Redline, Susan, National Heart, Lung and Blood Institute (NHLBI) of the National Institutes of Health, 2010.
 Marin JM, Carrizo SJ, Vicente E, Agusti AG, Long Term cardiovascular outcomes in men with obstructive sleep apnoea-hypopnoea with or without treatment with continuous positive airway pressure: an observational study. Lancet 2005;365:1046-1053.
 Gami AS, Howard DE, Olson EJ, Somers VK. Day-night pattern of sudden death in obstructive sleep apnea. N Engl J Med, 2005; 352:1206-1214.
 Reichmuth KJ, Austin D, Skatrud JB, Young T. Association of sleep apnea and type II diabetes: a population based study. Am J Respir Crit Care Med 2005;172:1590-1595.
 Punjabi N, Caffo B, Goodwin J, Sleep-disordered breathing and mortality: A prospective cohort study, National Heart, Lung and Blood Institute (NHLBI) of the National Institutes of Health, open access journal, PLoS Medicine, August 18, 2009.