tage 2 (NREM2 or N2) is the first unequivocal stage of sleep, during which muscle activity decreases still further and conscious awareness of the outside world begins to fade completely. If any sounds are heard, the sleeper is not able to understand their content at this point. Brain waves during stage 2 are mainly in the theta wave range (as in stage 1 sleep), but in addition stage 2 is also characterized by two distinguishing phenomena: sleep spindles (short bursts of brain activity in the region of 12-14 Hz, lasting maybe half a second each, also known as sigma waves) and K-complexes (short negative high voltage peaks, followed by a slower positive complex, and then a final negative peak, with each complex lasting 1-2 minutes) - see the diagram at right. Together, these serve to protect sleep and suppress response to outside stimuli, as well as to aid in sleep-based memory consolidation and information processing. Because sleepers pass though this stage several times during the night, more time is spent in stage 2 sleep than in any other single stage, and it typically constitutes about 45%-50% of total sleep time for adults (or even more in young adults).