历届高考完形填空语篇荟萃(28)

（单词翻译:单击）

(二十八)

At the beginning of this century, medical scientists made an interesting discovery: we are built not just of flesh（肌肉）and blood but also of time. They were able to show that we all have a 'body clock' inside us, which controls the rise and fall of our body energies, making us different from one day to the next.
The idea of a 'body clock' should not be too surprising since the lives of most living things are controlled by the 24-hour night -and-day cycle（循环）. We feel tired and fall asleep at night and become lively and energetic¹ during the day. If the 24-hour cycle is disturbed most people experience unpleasant² feelings. For example, people are not used to working at night can find that lack of sleep causes them to perform badly at work.
As well as the daily cycle of sleeping and waking, we also have other cycles which last longer that one day. Most of us would agree that we feel good on some days and not so good on others; sometimes our ideas seem to flow and at other times, they just do not exist. [NMET2000]
简析：说明文文体，阐述生物钟对我们的影响。