As congestion builds on one street relative to another, it adjusts traffic-light cycles to give more green time to the congested lanes.

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Light and dark cycles cause changes in levels of brain chemicals such as melatonin.

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The mask is designed so that light passing through its thickest region is slowed by exactly two cycles compared with that passing through its thinnest.

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In another experiment the researchers wake up sleeping mice by sending a light signal to the hypothalamus, the brain's controller of sleep-wake cycles.

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