不是 … ，就是 … (either … or …)

Sleeping or reading — no third option

• no不是bú shì … ，exactly就是jiù shì … = "either …, or …" — it's one of the two.
• he / him他tāno不是bú shìat / in在zàito go to bed睡觉shuì jiào，exactly就是jiù shìat / in在zàito read看书kàn shū = "he's either sleeping or reading."
• It narrows things to exactly two possibilities.

no不是bú shì + A，exactly就是jiù shì + B

• no不是bú shì…exactly就是jiù shì… says the truth is A or B, nothing else.

he他tāeither不是bú shì(doing)在zàisleep睡觉shuì jiào，or就是jiù shì(doing)在zàiread看书kàn shū。

• no不是bú shìat / in在zàito go to bed睡觉shuì jiào，exactly就是jiù shìat / in在zàito read看书kàn shū = "he's either sleeping or reading."

Recap

Read the full reference: HSK 5 grammar — Sentence patterns.