Enough sleep is important to health. The amount of sleep needed __31________ on the age of the person and the conditions in ___32_______ sleep takes place. The young may need more sleep than ___33_______ old, but usually eight hours are enough for the health of grown-ups. Some can do with __34________ than this amount, but others may need more. Every person knows his own need. ___35_______ is then a matter of good judgment to satisfy his need. Sleep __36________ always be enough to make one bring back his __37________ (strong) and get ready for a day’s work.

Fresh air is necessary to sound sleep. So it is not __38_________ reason for some people to think that it is practicable to sleep in the open air. A bath at bedtime, __39________ hot nor cool but of body temperature, may be helpful to sleep. Sleep-producing drug(帮助睡眠的药物) should never be taken except when ___40_________ (suggest) by a doctor.

选用电刷时应注意哪些性能指标？