No Stupid Questions

It starts with libations and food offerings.

Sweets were fed to the gods in ancient Mesopotamia and ancient India[7] and other ancient civilizations.[8] Herodotus mentions that Persian meals featured many desserts, and were more varied in their sweet offerings than the main dishes

https://en.wikipedia.org/wiki/Dessert#History

The Romans continued the practice.

https://en.wikipedia.org/wiki/Ancient_Roman_cuisine#Desserts

back to the main dessert article:

Europeans began to manufacture sugar in the Middle Ages, and more sweet desserts became available.[14] Even then sugar was so expensive usually only the wealthy could indulge on special occasions. The first apple pie recipe was published in 1381;[15] The earliest documentation of the term cupcake was in "Seventy-five Receipts for Pastry, Cakes, and Sweetmeats" in 1828 in Eliza Leslie's Receipts cookbook.[16]

And then there's this guy:

Evidence for the domestication of the cacao tree exists as early as 5300 BP in South America, in present-day southeast Ecuador by the Mayo-Chinchipe culture, before it was introduced to Mesoamerica.[8] It is unknown when chocolate was first consumed as opposed to other cacao-based drinks, and there is evidence the Olmecs, the earliest known major Mesoamerican civilization, fermented the sweet pulp surrounding the cacao beans into an alcoholic beverage.[9][10]

Chocolate was extremely important to several Mesoamerican societies,[11] and cacao was considered a gift from the gods by the Mayans and the Aztecs.[12][13]

https://en.wikipedia.org/wiki/Chocolate#History

Now as to "why before bed"? It's become a practice. But here's the thing: nobody is making you eat a sweet nor at a particular time of day.

In the 80s it was rare to see people drinking water, except for "health food nuts". It was far more common to see soft drinks/sodas. Over the years, society has become more accepting of drinking water. You didn't have "hydrohomies" in the 80s. Be the change you want to see in the world.