Definition: who sleeps dines

This is an interesting expression, not because of its meaning, which would be hard to convince a hungry person of, but because of the different theories about its origin.
On the one hand, many claim that it dates back to the Middle Ages, when travellers who wanted to sleep in an inn were also forced to dine there, or risk being refused lodging.
In other words: "if you eat, you sleep; if you don't eat, you leave!", which is now known as forced joint sales and is no longer permitted (in theory).
The saying dates back to the 18th century. It was posted on the walls of inns for travellers to see: those who wanted to rent a room had to dine there. Scientifically, it is rather the opposite – those who dine sleep – that would be logical.
On the other hand, we have Alain Rey, a distinguished linguist, who completely ignores this hypothesis and tells us that this proverb comes from the ancient belief that "sleep nourishes those who have nothing to eat", expressed by the Greek Menander, author of, among other things, "sleep brings counsel", but also of a quote that is worth reflecting on: "The earth and the sea produce a great number of ferocious animals, but woman is the greatest ferocious beast of all." This explanation is not repeated by other lexicographers in the sources available to us.
So who is right, the scholar who puts forward an opinion that he seems to be the only one to defend, or the masses who propagate another?

Note that even if Menander did express this thought, there is nothing to prevent the innkeepers of yesteryear from having practised forced sales. The two explanations are therefore not incompatible.
From a historical point of view, therefore, the saying "who sleeps dines" has a logical explanation. From a medical point of view, it might be better to say "who dines sleeps". Because contrary to what the saying suggests, it is better to dine before sleeping!
Eating makes you sleepy
It is true that we expend less energy at night than during the day, but this expenditure is not zero. The brain needs sugar constantly. In fact, our body makes this clear to us: it is more difficult to fall asleep when we are hungry.

And if hunger strikes during the night, it wakes us up, with the risk of causing discomfort related to hypoglycaemia in vulnerable individuals. Animal experiments even show that fasting can completely suppress sleep.
That's not all. The volume and nature of meals influence the duration and distribution of the different phases of sleep. Rats fed a rich diet sleep longer. In humans, the absorption of carbohydrates increases the tendency to feel drowsy within 30 to 60 minutes after ingestion. Conversely, a protein-rich meal seems to improve psychomotor performance.
Lack of sleep leads to overeating
Why? A meal rich in carbohydrates triggers the secretion of insulin, which in turn increases serotonin levels. This substance accelerates the feeling of satiety, but it also plays a role in falling asleep. In fact, there are more obese people among those who sleep less.
This is normal, as reduced sleep time is associated with a decrease in the satiety hormone, serotonin. Conversely, a protein-rich meal reduces serotonin synthesis and thus helps maintain alertness.
Overeating causes poor sleep
To all this, we could add "those who eat too much sleep poorly". Indeed, sleep requires a relatively low internal temperature. If the evening meal is very large, digestion, which increases body heat, takes longer and compromises a good night's rest. For the same reason, it is best to wait at least 1 hour 30 minutes after eating before going to bed.
In short, the saying, updated in light of today's knowledge, becomes "Those who dine properly sleep well".