Learn Hardcore German: Ich schlafe nachts gut.

German main clauses follow the verb-second (V2) rule. The finite verb must occupy the second position. Here, Ich (subject) is first, schlafe (verb) is second, and then the adverbs follow.

What part of speech is nachts, and why isn’t there a preposition like in or am?

nachts is an adverb meaning “at night.” It’s a fixed adverbial form that doesn’t require a preposition. You could also say:

in der Nacht (“during the night,” more specific)
bei Nacht (poetic/formal “by night”)
But for general statements, nachts is shorter and more idiomatic.

Could I say Ich schlafe gut nachts instead of Ich schlafe nachts gut?

German prefers the time–manner–place order for adverbs. Here:

nachts (time)
gut (manner)
So Ich schlafe nachts gut is correct. Swapping them (Ich schlafe gut nachts) sounds awkward and breaks that natural sequence.

What’s the difference between nachts, in der Nacht, and bei Nacht?

nachts: general, habitual “at night.”
in der Nacht: more specific “during the night,” often referring to a particular night or event.
bei Nacht: literary or formal “by night,” less common in everyday speech.

How is gut functioning here? It’s an adjective, right?

Yes, gut is an adjective used adverbially to modify the verb schlafe (“sleep”). In German, adjectives often serve as adverbs without any change in form: schlafe gut = “sleep well.”

How would I negate the sentence to say “I don’t sleep well at night”?

Place nicht before the adverbial you want to negate. For example:

Ich schlafe nachts nicht gut. (“I don’t sleep well at night.”)
You can also front nachts for emphasis:
Nachts schlafe ich nicht gut.

Can I start the sentence with nachts and say Nachts schlafe ich gut? Does the meaning change?

Yes, you can front nachts: Nachts schlafe ich gut. The meaning stays the same (“I sleep well at night”), but the emphasis shifts to time (i.e. “It’s at night that I sleep well”).

Why is it ich schlafe and not ich schlaf?

In standard written German, the first-person singular of schlafen is ich schlafe; the -e ending is required. In casual spoken German you might hear ich schlaf, but that’s colloquial and not used in formal writing.