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.
This fact shows suppression of bone marrow activity, as a hematological sign specific for pernicious anemia and radiation sickness.A leukocyte count above 25 to 30 × 109/L is termed a leukemoid reaction, which is the reaction of a healthy bone marrow to extreme stress, trauma, or infection. It is different from leukemia and from leukoerythroblastosis, in which either immature white blood cells (acute leukemia) or mature, yet non-functional, white blood cells (chronic leukemia) are present in peripheral blood. Classification
Leukocytosis can be subcategorized by the type of white blood cell that is increased in number. Leukocytosis in which neutrophils are elevated is neutrophilia; leukocytosis in which lymphocyte count is elevated is lymphocytosis; leukocytosis in which monocyte count is elevated is monocytosis; and leukocytosis in which eosinophil count is elevated is eosinophilia.An extreme form of leukocytosis, in which the WBC count exceeds 100,000/µL, is leukostasis. In this form there are so many WBCs that clumps of them block blood flow. This leads to ischemic problems including transient ischemic attack and stroke. Causes
Leukocytosis is very common in acutely ill patients. It occurs in response to a wide variety of conditions, including viral, bacterial, fungal, or parasitic infection, cancer, hemorrhage, and exposure to certain medications or chemicals including steroids.