In contrast, Manser says, a person's passive vocabulary consists of the words whose meanings they knowâso that they do not have to look the words up in a dictionaryâbut which they would not necessarily use in ordinary conversation or writing (The Penguin Writer's Manual, 2004).
Sleep Apnea, Night Terrors, and Nightmares. Sleep apnea may also be to blame for your nightmares. During sleep apnea, your brain senses the low oxygen and realizes that you are under a very real threatâyou are being choked or smothered by your own bodyâand if nothing is done you will die.
Under normal conditions, oxygen levels during sleep are maintained in a normal range. However, certain medical problems can lower your blood oxygen below normal levels, and this effect can worsen during your sleep. This can present a real risk for sudden death if not treated. Chronic low blood oxygen level (hypoxemia) at night can also lead to health problems.