Sleep involves great changes in brain activity. Until the 1950s it was generally believed that the brain essentially shuts off during sleep, but this is now known to be far from true; activity continues, but patterns become very different. There are two types of sleep: REM sleep (with dreaming) and NREM (non-REM, usually without dreaming) sleep, which repeat in slightly varying patterns throughout a sleep episode. Three broad types of distinct brain activity patterns can be measured: REM, light NREM and deep NREM. During deep NREM sleep, also called slow wave sleep, activity in the cortex takes the form of large synchronized waves, whereas in the waking state it is noisy and desynchronized. Levels of the neurotransmitters norepinephrine and serotonin drop during slow wave sleep, and fall almost to zero during REM sleep; levels of acetylcholine show the reverse pattern.
During what stage of sleep do serotonin and norepinephrine levels drop?
slow wave sleep

By the end of the 12th century, samurai became almost entirely synonymous with bushi, and the word was closely associated with the middle and upper echelons of the warrior class. The samurai were usually associated with a clan and their lord, were trained as officers in military tactics and grand strategy, and they followed a set of rules that later came to be known as the bushido. While the samurai numbered less than 10% of then Japan's population, their teachings can still be found today in both everyday life and in modern Japanese martial arts.
What were samurai affiliated with?
a clan

Travel and tourism continue to be extremely important for Portugal, with visitor numbers forecast to increase significantly in the future.[citation needed] However, the increasing competition from Eastern European destinations continues to develop, with the presence of similar attractions that are often cheaper in countries such as Croatia. Consequently, it has been necessary for the country to focus upon its niche attractions, such as health, nature and rural tourism, to stay ahead of its competitors.
What attractions does Portugal have to offer tourists?
health, nature and rural tourism