Read the text and complete the sentences with the correct form of the given words

Everyone knows about the standard 8-hour sleep. But why is it exactly 8 hours?

In recent years there were plenty of studies conducted in different countries to determine how often diseases affect different groups of the population. It might be [ ], but all of them have come to the same conclusion: people who suffer from lack of sleep, as well as those who sleep too much, are more likely to become ill. Therefore, such people usually live less.

What happens?

If an adult person sleeps too little (less than 6 hours), they cannot be fully [ ]. And even if an adult sleeps too much (more than 10 hours), this is bad too, as it increases the risk of becoming ill. In both cases, there is a [ ] effect on the organism. First of all, brain function is getting worse, as well as the person's mood. Moreover, immunity reduces gradually. What is even more [ ] is that the risk of getting heart problems and obesity rises. In one recent research, participants who had less than seven hours of sleep were three times more likely to catch a cold than those who had seven or more hours of sleep.

And what about teenagers?

Sleep experts believe that teenagers need to sleep up to 10 hours a day. However, in reality, almost half of teens sleep significantly less. Nowadays, we have more variety of [ ] entertainment than ever before — as a result, it is tempting for teenagers to stay awake more. Bedrooms are no longer just rooms to relax. Now there are lots of distractions such as laptops, mobile phones, video games and others. All these [ ] devices make the process of going to bed more complex. Also, every year more and more parents become [ ] about the situation.