Most physiological functions like pulse, B.P. and respiration are markedly reduced during the phase of NREM sleep. REM sleep is characterized by an increase in physiological activity i.e. an increase in pulse, B.P., and respiration. REM and NREM sleep alternate with each other. The cyclical nature of sleep is regular and reliable, REM period occurs about every 90 to 100 minutes during the night. Sleep regulation is not by a simple control centre but by a number of interconnecting centres.
Many studies support the role of a number of chemical substances in sleep regulation.
Dreams occur during the REM phase of sleep, which is generalized and common to all human beings. Hence everybody's sleep has this dream state or REM sleep state. In case your sleep is interrupted in the REM phase, you have a clearer memory and imagery of the dream. Sleep requirement differs in different people. Short sleepers require fewer than 6 hours of sleep each night whereas long sleepers are those who sleep more than 9 hours each night in order to function adequately.