Sleep is part of a person's daily activity cycle. There are several different stages of sleep, and they too occur in cycles.

睡眠是人每天日常活动循环的一部分。人的睡眠分几个阶段，而这些阶段也是循环发生的。

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During the 21st solar activity cycle the HXEBS aboard SMM satellite and the HXM on HINOTORI spacecraft detected several thousand hard X-ray solar flares.

在第21周太阳活动峰年期间，“太阳峰年”卫星和“火鸟”卫星上的硬x射线爆发探测器探测到数千个硬x射线太阳耀斑。

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Mice usually have a characteristic daily activity cycle that lasts 23.6 hours. But after a week on high-fat chow, the experimental mice had a pattern about 20 minutes shorter than normal.

一般情况下小鼠特征性的周期活动每天持续23.6小时，但是高脂饮食后一周实验组小鼠的这种活动比正常组减少了20分钟。

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Climate scientists have already looked at changes related to Sun spot activity - a cycle of approximately 11 years - and long-term changes in the Sun's brightness, which has a cycle that lasts for centuries.

BBC: Viewpoint: The Sun and climate change

But in the current cycle, sunspot activity has collapsed.

FORBES: To The Horror Of Global Warming Alarmists, Global Cooling Is Here

Sunspot activity waxes and wanes on the sun in a 11-year cycle, and we are approaching the peak of sunspot activity, probably reaching it in 2013, according to Hesse.

FORBES: Huge Solar Flares Could Spell Catastrophe for Earth