What aspect of circadian rhythms might explain why we tend to feel so tired at night?

There is an organ in the brain sensitive to light changes, which then affects our sleepiness.
This sensitivity to light changes will determine our response to melatonin that influences our level of 'tiredness'.
The more melatonin that we have in our body, the more likely we feel sleepy and fall asleep.

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Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.