We most often associate “Circadian Rhythms” with sleep, but our body’s internal clock actually manages quite a few processes. In fact, our sleep cycles may fall out of synch with our circadian rhythms. (Most commonly, we may go to sleep after our body has started slowing things down and awake after it starts speeding things up.) According to some researchers this may be a leading factor in depression. In a 1979 study, patients who experienced depression were found to achieve REM sleep* almost immediately after going to bed, and to wake up many hours later. When they went to bed sooner (delaying the onset of REM) they often experienced fewer depressive symptoms.

This may help explain a strange fact of life at high latitudes: the increase in depression (and suicides) during the spring season. Communities north of the Arctic Circle experience two unique seasonal phenomena: “midnight sun” and “polar night.” “Midnight sun” refers to the period when the sun is visible for 24 hours a day (which lasts from late-April to late-August); “polar night” refers to to similar periods of total darkness during the winter. It’s easy to imagine being depressed after weeks without sunlight, but the incidence of suicide is actually higher while the midnight sun is shining. Circadian rhythms give us a clue to why this might be the case: your sleep cycle is much more likely to fall out of kilter with your internal clock when you have no environmental queues to help you. Devoted Intellectuals are certainly familiar with late nights of reading and restlessness, but those nights are actually a lot later when the sun shines straight through.

* For more on REM, and sleep in general, see the entry for Week 29, Day 4.

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A daily dose of the Devotional isn’t the only way to increase brain activity. For 15 more mental exercises (from eating curry to playing pool), click here.

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Thomas Jefferson did not end slavery in the United States, though he did make an attempt. In the first draft of the Declaration of Independence, Jefferson condemned the act of slavery in rather curious language. He called it, “this piratical warfare, the opprobrium of infidel powers.” Pirates? Infidels? These seem like odd terms with which to describe plantation owners. But Jefferson wasn’t talking about the South. He was talking about North Africa.

In the two centuries before the Declaration, over a million Europeans and Americans were taken as slaves by North African pirates, or the “Barbary Pirates” as they were known at the time. Jefferson took a particularly hard line on this. He was determined to end the practice and, as president, he built the U.S. Navy almost exclusively for that purpose, a fact is still commemorated in the Marines’ Hymn:

From the Halls of Montezuma
To the Shores of Tripoli
We fight our countries battles
On the land as on the sea

The first major war of an independent United States was fought on the shores of Tripoli, and Algiers and Morocco and Tunis. By 1805, the war was over and Americans were no longer threatened with slavery in the Mediterranean. It would take another 60 years, and another bloody war, before Americans were freed from slavery at home.

For more on the topic, read this survey of new books on the Barbary conflict.

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