Where exactly are you hearing this phrase used? I've never heard it, and an informal poll of the mod team also indicates it certainly doesn't have wide credence. Google turns up only one hit which uses it... this thread.
The Spartans didn't use our 24-hour system or anything like it to indicate time, so it's impossible to answer your question to any degree of precision. On the other hand, the origin of the saying seems pretty clear. (Be warned: this post starts off pretty lighthearted, but swerves suddenly into discussing systemic child abuse. I apologise for this – the question shall receive its answer.)
We know that full Spartan citizens were required to spend all their evenings in the company of the other men of their syssitia (variably named in Greek and variously translated “mess group”, “tent group” or similar). These groups formed the units around which Spartan life was organised. They are a formalised version of the hetaireia (companionships) that were an element of leisure-class life throughout the Greek world: closed circles of wealthy men who gathered for symposia (drinking parties) in which food, drink, poetry, dance and debate were shared. Where other Greek societies accepted the existence of hetaireia as a way for the elite to cement social networks and exchange (often anti-democratic) ideas, the Spartans positively mandated that every citizen had to be part of a syssitia and had to partake in meals, songs, traditions, exercise and war together with the same small group of associates. This was a way to enforce conformity to the values of Spartan society, to regulate the way Spartiates used their free time, and to ensure that the experience of citizenship was outwardly the same for all Spartiates.
The result of the legal requirement to dine with the tent group was that every Spartiate spent every night at a drinking party. The symposion was stereotypically a bawdy affair; it was common to engage in rounds of “bottoms up” drinking to the health of the host, the gods, the musicians and dancers and rhapsodes offering entertainment, or whoever else came to mind. We know of Classical Greek drinking songs and drinking games. We have vase paintings of men throwing up (link NSFW) because they've had too much; such pictures were sometimes humourously painted at the bottom of a wine cup, so that they would appear as the drinker got literally and figuratively “in their cups”. In short, if the dinners of Spartan tent groups were too much like drinking parties of hetaireia elsewhere, the famed Spartiates might have spent most of their time either plastered or nursing a hangover.
The Spartans, therefore, decided it would be wise to temper the excesses of their drinking parties. They allegedly taught their children about the dangers of drunkenness by getting their helots trashed and forcing them to perform humiliating dances. They rationed the amount of wine allowed per man on campaign, and at home they banned the practice of drinking rounds to people's health. However, they also put in place a very simple rule: they forbade citizens of military age (20-60) from carrying lamps or torches when they made their way home from their messes. Those who were too wasted would inevitably get lost or fall into a ditch somewhere, to the general merriment of the other members of their tent group. To avoid the humiliation, it was hoped, Spartiates would keep careful track of their consumption to make sure they could still find their way home (Xenophon, Constitution of the Lakedaimonians 5.7).
From this anecdote, we learn one thing about bedtime for Spartiates: it was already dark when they went home. It seems likely that the saying you're citing is derived from this simple observation. That said, the time indicated is pretty lenient. At the latitude of the Mediterranean, dusk occurs well before 10pm at all times of the year, so Spartiates wouldn't have had to stay out that late in order to have to make their way back in the dark. The latest sunset, at around the summer solstice, is at 9pm.
The more interesting question is why there would be an early twentieth-century German saying of this kind, referring to the lifestyle of Spartiates. Presumably the ways of the Spartans as recorded by Xenophon in the 4th century BC would not exactly have been common knowledge among the German population. But, as the fascinating research of Helen Roche has shown, there was one group of Germans who were obsessed with the Spartan Way, emulated the perceived Spartan ethic in every way, and self-identified proudly as “Spartan youths” (Spartanerjunge): the young cadets of the Prussian officer schools.^1
These schools, scattered about the country that formerly made up the Prussian kingdom, offered an absolutely ruthless education, intending to create tough and unflinching officers by inflicting upon its cadets a regime of exposure, deprivation, undernourishment, brutal corporal punishment, and systemic hazing and bullying. Roche cites a former cadet visiting a famously harsh British boarding school and remarking that the softness of British officers was not surprising given how comfortable their upbringing was. The children sent into the Prussian cadet schools were accustomed to humiliation and suffering to the point where they made their stoic endurance of such horrors into a point of pride; it was common for cadets to inflict pain upon themselves just to show how much they could take. The identity these cadets adopted in order to endure constant deprivation and abuse was that of “Spartan youths” – since the Spartans, according to late sources like Plutarch, subjected their children to a similar education in order to harden them into the finest warriors of the ancient world. In this way the cadets could persuade themselves that what they suffered was for the greater good; it would make them into the sort of men who would fight to the death at Thermopylai.
While the adoption of this identity was of course encouraged by the teachers at the Prussian cadet schools, there doesn't seem to have been any official impulse to spread this notion among the cadets. In other words, the idea of adopting “Spartan” ethics must have arisen among the cadets themselves, gradually spreading into the culture of Prussian officers as former cadets grew up and took office in the armed forces. This means that the view of Sparta held by these would-be Spartan youths is based on what few sources and secondary works they could have had access to. It's clear that the notion of Spartan-ness held by the cadets is pretty narrow: it's based pretty much exclusively around Plutarch's description of the late, re-reformed agoge and around the legend of Thermopylai. It ignores much else that is known about Spartan society. Roche notes that the young cadets seem to have gotten their understanding of Sparta mainly from Plutarch's Life of Lykourgos, and little else. But it so happens that this source repeats the anecdote found in Xenophon:
After drinking moderately, they go off home without a torch; for they are not allowed to walk with a light, either on this or any other occasion, that they may accustom themselves to marching boldly and without fear in the darkness of night.
-- Plutarch, Life of Lykourgos 12.7
Now, I obviously don't actually know the context of the saying, or who would have used it, or when. But based on the simple information that it may have been a local German saying from the beginning of the twentieth century, it may well be that this was an aspect of the common culture of the Prussian cadets, who sought to emulate in every way the Spartans they so admired.
Hi everyone!
Since this thread is taking off, I want to post a reminder about two things:
also
Thank you!
If I can piggyback this question ever-so-slightly, there's an anecdote in the field of circadian rhythm studies that medieval Europeans practiced a biphasic sleep schedule (4 hours asleep, about an hour awake, another 4 hours asleep).
Did this also apply to the Spartans (or other Greek societies if that's not too far off-topic)?
