From what I've read about war, it seems like an environment in which people get badly sleep-deprived and thus fall asleep involuntarily all the time; so if you shot soldiers for sleeping at their posts, wouldn't you end up executing half your army?
Was Bierce exaggerating? Was this a punishment that was on the books but rarely meted out in practice? Or were the armies of the US Civil War just unusually draconic in their punishments? Also, do we have any numbers -- how many men were actually executed for this reason during the war?
Many thanks for any answers you guys can give!
It was actually quite a common practice during the Civil War. I can only answer for the Union side, not the Confederacy. There was a significant debate about whether it was "necessary to maintain military discipline" (as believed by most generals as well as notably Secretary of War Edwin Stanton) or generally (in cases of sleep from weariness instead of willful abandonment) draconian as was the belief held by the one man able to pardon the offense, namely President Lincoln.
One particular occurrence I recall was of a soldier who covered for a sick friend by taking his watch one night and then had his name called to stand watch the next. Utterly sleep deprived he fell asleep and was (despite his being an exemplary soldier) sentenced to death for dereliction of duty. Lincoln ended up pardoning him and basically reprimanded the military for their complete lack of common sense in the matter in letting a man keep watch two nights in a row.
It was a hotly debated topic between Lincoln and the military brass, especially Stanton (who Lincoln respected and deferred to ask much as an cabinet member except maybe Secretary of State William Seward) on the subject of seemingly excessive punishments like death for falling asleep. Lincoln was not a complete pushover though; he granted many pardons, but he also allowed many others to be punished. He was so affected by it that on the days the sentences were carried out he would attempt to be somewhere that he wouldn't hear the gunshots of the executioners in the distance.
tl;dr Yes it was very much a common occurrence, but it was also controversial.
Source: "Team of Rivals: The Political Genius of Abraham Lincoln" by Doris Kearns-Goodwin
For supporting information, Article 46 of the Articles of War states:
Any sentinel who shall be found sleeping upon his post, or shall leave it before he shall be regularly relieved, shall suffer death, or such other punishment as shall be inflicted by the sentence of a court martial.
The Articles of War were established on April 10, 1806 and remained unchanged throughout the Civil War. The Lieber Code also applied to the Civil War, but makes no mention of punishment of specific offences. It relates to the conduct of an army in general, and was a sort of precursor to the Geneva Conventions.
However, I have no information on whether or not the punishment was routinely or ever carried out.
I should comment that falling asleep on sentry is an EXTREMELY serious offense, even today. Because by falling asleep when you're supposed to be guarding all of your comrades, you're effectively sentencing them all to death. In many militaries right now, sleeping on sentry is an offense punishable by death. It is absolutely no joke and is one of the more serious offenses you can commit before crossing into war crimes.
For a bit of context, here are some military's current laws and punishments for sleeping on sentry.