Learn Hardcore Turkish: Aniden esnedim; uyumayı ertelememek lazım. - I yawned suddenly; we shouldn’t postpone sleeping.

Çünkü: Aniden esnedim, çünkü uyumayı ertelememek <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/turkish/verbs/necessity-gerek">lazım . (I yawned suddenly because…)
Demek ki: Aniden esnedim; demek ki uyumayı ertelememek lazım. (So it seems we shouldn’t postpone sleep.) The semicolon in your original already implies a related thought without an explicit connector.

Aniden esnedim (most natural: adverb before the verb).
Ben aniden esnedim (adds an explicit subject for emphasis).
Esnedim aniden (possible for emphasis, but less neutral). Synonyms: birden, birdenbire, ansızın (register varies slightly: ansızın is a bit literary).

Why isn’t there a subject pronoun like ben before esnedim?

Turkish verbs show the subject in their endings. Esnedim already means I yawned; the suffix -dim marks first-person singular past. You add ben only for emphasis or contrast.

How is esnedim formed, and how would I change the tense or make it negative?

Root: esne- (to yawn)
Past tense: -di
1st singular: -m So: esne-di-m → esnedim.
Other forms:
Negative: esnemedim (I didn’t yawn)
Present continuous: esniyorum (I am yawning)
Aorist (habit): esnerim (I yawn, habitually)