Ⅰ.七选五
A lack of sleep causes many problems that greatly affect the quality of life.While no one knows why we sleep, it is a universal biological need.__1__
How much sleep do I actually need?
A study in Finland found that the amount of sleep that relates to the fewest sick days was 7.63 hours a night for women and 7.76 hours for men.__2__ When we get sleep fewer than seven hours or our sleep often falls below six hours, we are at an increased risk of health problems.
__3__
Today, some people practice “polyphasic (多阶段的)sleep”， based on the idea that by dividing your sleep into parts，you can get away with less of it.Professor Dinges concludes it's possible to train oneself to need less sleep in short periods.__4__ And he notes that even for those who can function well on less sleep, we still don't know how the practice might be affecting chemical processes, mood, and other factors.
I can't sleep.Is my phone really keeping me up?
When light enters your eye, it hits your retina (视网膜) to send signals to the hypothalamus (下丘脑), which controls sleep cycles.When your retina takes in less light, your hypothalamus assumes it's time to sleep and it'll signal the pineal gland (松果腺) to make you sleepy.And when the hypothalamus senses light，it tells the pineal gland to stop its work.__5__ Some research suggests that the light from phones and tablets are especially influential in human sleep cycles.
A．Adequate sleep can benefit us a lot.
B．Can I train myself to need less sleep?
C．How can I make myself sleep less?
D．No animal with a brain can survive without it.
E．This is why we're told to minimize screen time before bed.
F．And most adults function best after seven to nine hours of sleep a night.
G．But he adds it doesn't seem possible to train oneself to need less sleep per 24hour cycle.
