grammar - Is 寝る a stative or active verb?

Looking at past questions I am still confused with the answers given:

1. The answer to this post suggests that 寝る is a continuation of some state.

• 昨日も全然寝てない。 Yesterday I didn't sleep at all.

Instead of

• 昨日も全然寝てなかった。 Yesterday I didn't sleep at all.

2. The answer to this post suggests that 寝る is an active verb

• 寝てろ Keep sleeping (keep doing the action of sleeping)

---

I want to confirm my understanding of active vs stative verbs:

Active

書いている - (I) am writing [present progressive]

書いていない - (I) am not writing [negative present progressive]

書いていた - (I) was writing [past progressive]

書いていなかった - (I) was not writing [negative past progressive]

Stative

帰っている - (I) have (already) came home [present progressive]

帰っていない - (I) I have not come home (but I might be in the middle of coming home) OR I didn't come home. (also in the past?) [negative present progressive]

帰っていた - (I) I had come home (sometime in the past I came home) [past progressive]

帰っていなかった (I) had not come home (sometime in the past I didn't come home) [negative past progressive]

---

I escpecially have trouble differentiating (for stative verbs) between 帰っていない and 帰っていなかった。

Because of that I also cant understand why 昨日も全然寝てない。 is not 昨日も全然寝てなかった。If it was a stative verb.

If 寝る is an active verb in this scenario it also wouldn't make sense at all because I would translate it as "yesterday I am not sleeping".

This post may be related but I only see examples where it is used as an answer to a question.

I would appreciate an explanation of the grammar behind this.

Answer

寝る works as both. See the example of 溶ける in this answer. 寝ている/寝てる can mean both "is sleeping" (present progressive) and "have slept" (present perfect). 寝ていた/寝てた can mean both "was sleeping" (past progressive) and "had slept" (past perfect). It depends on the context.

When you talk about something in the past, the difference between 寝た and 寝ている can be sometimes blurry, and they are often interchangeable. For instance, if someone in front of you looks sleepy, you can say both "昨日あまり寝てないの?" and "昨日あまり寝なかったの?". But 寝ている tends to be used in relation to the current situation, so 寝てないの may be chosen more often in this case. Likewise, before a boring but important event, you would usually choose "昨日ちゃんと寝てる?" rather than "昨日ちゃんと寝た?" See also: Why is a verb in the past (た形) contradicted with ～ていない?

Imperative 寝て(い)ろ always has a progressive meaning, because making a command about something in the past does not make sense (You cannot say "Have slept!" in English, either.)

Past perfect 寝て(い)なかった ("had slept") is relatively uncommon, but it's used when you talk about it in relation to another event in the past.

• 昨日よく寝ていなかったので今朝の試験に落ちた。
  
  I failed the examination in this morning because I had not slept well."


• 夫は家に帰っている。
  My husband has arrived home. (He is at home now)


• 私が家に帰ったとき、夫ももう帰っていた。
  When I arrived home, my husband had arrived home, too. (Where he is now is not important)

EDIT: Here is the basic difference between present and past perfective aspect. The eye icon indicates what you are describing. Past perfective aspect describes a continuation of state until some point of interest in the past; its current status is not important. 寝ていた works just like this, too, but it can also mean progressive "was sleeping".