 The level of activity in the locus coeruleus correlates broadly with vigilance and speed of reaction . LC activity is low during sleep and drops to virtually nothing during the REM ( dreaming ) state . It runs at a baseline level during wakefulness , but increases temporarily when a person is presented with any sort of stimulus that draws attention . Unpleasant stimuli such as pain , difficulty breathing , bladder distension , heat or cold generate larger increases . Extremely unpleasant states such as intense fear or intense pain are associated with very high levels of LC activity . 
