Which stage(s) of sleep is (are) shown in the figure?

(Answer to Q. 1)  ANS: B
Stage 2 sleep is characterized by relatively low-voltage background electroencephalographic ac-tivity, sleep spindles and K-complexes, absence of eye movements, and tonic activity on an elec-tromyogram.

(Answer to Q. 2)  ANS: A
Normally, 4 to 6 cycles of sleep stages occur per night.

(Answer to Q. 3)  ANS: C
During a typical night of sleep, a normal adult sleeper cycles between non-REM sleep and REM sleep approximately every 90 to 120 minutes.

(Answer to Q. 4)  ANS: D
REM sleep is characterized by the following physiologic effects on respiration: irregular pattern of breathing; short, central apneas; variable and elevated PaCO2 (2-8 mm Hg above wakefulness); inhibited rib cage muscles; inhibited upper airway muscles; decreased chemoresponsiveness; and high threshold of arousability to respiratory stimuli.

(Answer to Q. 5)  ANS: D
Stage 2 non-REM sleep is identified on a sleeper's electroencephalogram by the appearance of sleep spindles and K-complexes. Relatively low-voltage, mixed-frequency waves occur during non-REM stage 1 sleep. Delta waves are present during slow-wave sleep or non-REM sleep stages 3 and 4.