Obstructive sleep apnea occurs when the opening to the upper airway is partial or completed block by the tongue, uvula or fatty tissue during the deeper stages of sleep. The condition tends to develop over time with the frequency of partial or complete blockages increasing. It is difficult to say when the occurrence of apneas (complete blockage) and hypopneas (partial blockage) is enough to cause excessive daytime sleepiness or other health issues such as cardiovascular disease, diabetes or depression.