Why can't researchers study REM sleep easily?

Sleep is an important part of your daily routine—you spend about one-third of your time doing it.  Quality sleep – and getting enough of it at the right times -- is as essential to survival as food and water.  Without sleep you can’t form or maintain the pathways in your brain that let you learn and create new memories, and it’s harder to concentrate and respond quickly.

Sleep is important to a number of brain functions, including how nerve cells (neurons) communicate with each other.  In fact, your brain and body stay remarkably active while you sleep.  Recent findings suggest that sleep plays a housekeeping role that removes toxins in your brain that build up while you are awake.  

Everyone needs sleep, but its biological purpose remains a mystery.  Sleep affects almost every type of tissue and system in the body – from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance.  Research shows that a chronic lack of sleep, or getting poor quality sleep, increases the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression, and obesity.

Sleep is a complex and dynamic process that affects how you function in ways scientists are now beginning to understand.  This booklet describes how your need for sleep is regulated and what happens in the brain during sleep.

Explanation:

https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep

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the quote relates to no dogs barked because the father and son learn a lot from each other. the theme is love because the father misses his son and they are trying to get to his mom where her dogs are barking.