hypo-, hyp-

(Greek: under, below, beneath; less than; too little; deficient, diminished; used as a prefix)

A diseased state of the blood in which the quantity of fibrin is below normal.
hypnagogue (s) (noun), hypnagogues (pl)
A hypnotic agent that induces sleep: Jack had problems with getting to sleep and sleeping through the night, so his doctor gave him a prescription for hypnagogue which should give him a good night's sleep!

A hypnagogue can be any agent causing sleepiness or drowsiness, like an uninteresting book or a boring movie!

hypoacidity (s) (noun), hypoacidities (pl)
Deficiency of normal acidity: Hypoacidity is a condition of having an insufficient amount of acidity in the gastric juice of a person's stomach.
hypoactive (adjective)
Less than normally active or under active: "The child's hypoactive behavior concerned the boy's parents and the doctor who examined him."
hypoactivity (s), hypoactivities (pl) (nouns)
Any abnormally decreased activity of the body or its organs; such as, the decreased cardiac output, thyroid secretion, or peristalsis: "The hypoactivities of the malnourished or starving children in areas of the world are obvious indications that they are going to die unless they are properly fed soon."
hypoacuity (s) (noun), hypoacuities (pl)
Decreased sharpness of sense perception: Since Jane noticed a change in her hearing, she decided to go to the otorhinolaryngologist for a checkup and found out that she couldn't hear as well as before, and he diagnosed her condition as being a case of hypoacuity.
hypoacusis (s) (noun), hypoacuses (pl)
Reduced or slightly impaired hearing which often involves neurosensory organs: In the early 1930s, when Bill realized that he was experiencing hypoacusis, he could not afford a hearing aid, so he put a small button on a string, put the button in his ear and the string in his pocket and, as a result, Bill noticed that people automatically spoke louder and so he had no trouble understanding them."
hypoadenia (s) (noun), hypoadenias (pl)
Any deficiency in the function of a glandular organ or tissue: The outdated term hypoadenia refers to the functional inadequacy of the activity of the glands in an organ or in the tissues of a person's body.
hypoalgesia (s) (noun), hypoalgesias (pl)
A decreased sensitivity to pain: Due to Linda's hypoalgesia, she didn't feel the cut on her finger which happened while getting dinner ready.
hypobaropathy (s) (noun) (no pl)
Sickness or disturbances experienced in high altitudes; high-altitude sickness: Hypobaropathy is due to reduced air pressure and oxygen intake.

Hypobaropathy is caused by reduced barometric pressure and not always distinguished from hypobarism.