However, this closure is still enough to cause a physiological effect i.e., an oxygen desaturation and/or an increase in breathing effort terminating in arousal. A Hypopnea Index (HI) can be calculated by dividing the number of hypopnea events during the sleep period by the number of hours of sleep. The Apnea-Hypopnea Index (AHI) is an index of severity that combines apneas and hypopneas. Combining them both gives an overall severity of sleep apnea including sleep disruptions and desaturations (a low level of oxygen in the blood). The apnea-hypopnea index, like the apnea index and hypopnea index, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep. Another index that is used to measure sleep apnea is the Respiratory Disturbance Index (RDI). The RDI is similar to the AHI, however, RDI also includes respiratory events that do not technically meet the definitions of apneas or hypopneas, such as a Respiratory Effort Related Arousal (RERA), but do disrupt sleep. Treatment
Obstructive hypopnea
One treatment for obstructive hypopnea is continuous positive airway pressure (CPAP). CPAP is a treatment in which the patient wears a mask over the nose and/or mouth. An air blower forces air through the upper airway. The air pressure is adjusted so that it is just enough to maintain the oxygen saturation levels in the blood. Another treatment is sometimes a custom fitted oral appliance.
Causes
Among the causes of hypopnea are:

anatomical defects such as nasal septum deformation or congenital narrowness of nasal meatus and the gullet
acute tonsillitis and/or adenoiditis
obesity or being overweight
neuromuscular disease or any condition that entails weakened respiratory muscles
hypoventilation syndromes involving compromised or failed respiratory drive
use of sedatives, for example sleeping pills
hazardous alcohol use
smoking
aging
others, most of which are also typical causes of airway obstruction, snoring and sleep apnea

Diagnosis
In the context of diagnosis and treatment of sleep disorders, a hypopnea is not considered to be clinically significant unless there is a 30% or greater reduction in flow lasting for 10 seconds or longer and an associated 4% or greater desaturation in the persons O2 levels, or if it results in arousal or fragmentation of sleep. The direct consequence of hypopnea (as well as apnea) is that the CO2 in the blood increases and the oxygen level in the patients blood decrease is proportionate to the severity of the airway obstruction. This disruptive pattern of breathing generates disruptive sleep patterns, the consequences of which being that those individuals may exhibit increased fatigability, lethargy, decreased ability to concentrate, increased irritability, and morning headaches. Basically, those individuals are extremely tired due to their inability to get a good nights sleep. Hypopneas can be either central i.e., as part of a waxing and waning in breathing effort, or obstructive in origin. During an obstructive hypopnea, in comparison to an obstructive apnea, the airway is only partially closed.