To neglect sleep and forget about eating
To be completely absorbed in one's work; to focus on something with single-minded devotion
Positive
这本书如此有趣, 竟使他废寝忘食 | Zhè bĕn shū rúcĭ yŏuqù jìng shĭ tā fèiqĭnwàngshí
The book is so interesting that it takes complete possession of him
[Source]
他们刻苦学习到了废寝忘食的程度 | Tāmen kèkŭ xuéxí dào le fèiqĭnwàngshí de chéngdù
They are so diligent in their studies that they neglect their sleep and meals
[Source]
To be wrapped up in one's work
None
I think another English equivalent might be "workaholic," but since its connotation is the opposite of this chengyu, I didn't include it in the "English Equivalent" section
This is an HSK chengyu
《列子》(Liè Zǐ), a set of Daoist texts said to be by the author of the same name. He is said to have lived during the Warring States period (475 - 221 BC)
When 孔子 (Kǒng Zǐ, Confucius) reached old age, he started traveling to lots of different places. When he was 64, he traveled to the kingdom of 楚 (Chǔ), where he was warmly received by an official of the town. The official knew that 孔子 was a great philosopher and statesman, and he knew that he had taught many outstanding students, but he didn't know very much about the man behind the reputation. As such, he asked 孔子's student, 子路 (Zǐ Lù), what 孔子 was really like. However, despite the fact that 子路 had been with his teacher for many years, he didn't know how to respond and therefore didn't utter a single word.
When 孔子 found out, he asked 子路, "Why didn't you tell him that I study so hard that sometimes I forget to eat? That I deeply enjoy imparting knowledge unto others and therefore don't worry about hardships? That as I strive for self-improvement, I sometimes even forget my own age?" These words would have demonstrated that 孔子 had a meaningful life.