<P> Dietary and nutritional choices may affect sleep duration and quality . One 2016 review indicated that a high carbohydrate diet promoted shorter onset to sleep and longer duration sleep than a high fat diet . A 2012 investigation indicated that mixed micronutrients and macronutrients are needed to promote quality sleep . A varied diet containing fresh fruits and vegetables, low saturated fat, and whole grains may be optimal for individuals seeking to improve sleep quality . High - quality clinical trials on long - term dietary practices are needed to better define the influence of diet on sleep quality . </P> <P> Research suggests that sleep patterns vary significantly across cultures . The most striking differences are between societies that have plentiful sources of artificial light and ones that do not . The primary difference appears to be that pre-light cultures have more broken - up sleep patterns . For example, people without artificial light might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours . </P> <P> The boundaries between sleeping and waking are blurred in these societies . Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep . </P> <P> Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods . In many nomadic or hunter - gatherer societies, people will sleep on and off throughout the day or night depending on what is happening . Plentiful artificial light has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced . In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case . </P>

Which of the following are proposed biological functions for why we need to sleep