Learn Hardcore Russian: Если я поздно ложусь спать, мне кажется, что я зря не дал себе поспать больше. - If I go to bed late, it seems to me that I foolishly didn’t let myself sleep more.

Ложусь by itself just means “I’m lying down” (no idea for what).
Ложусь спать is an idiomatic phrase meaning “I go to bed / I’m going to sleep”.
Сплю = “I am sleeping”, which describes the state of already being asleep, not the act of going to bed.

So ложиться спать is the standard way to say “to go to bed / to lie down in order to sleep”.

What is the difference between ложиться спать and идти спать?

Ложиться спать literally: “to lie down to sleep”. Focuses on the actual getting into bed / lying down.
Идти спать literally: “to go to sleep”. Focuses more on the decision and movement toward bed.

In many everyday contexts they’re interchangeable (both mean “go to bed”), but:

ложиться спать feels a bit more “physical”, about the action of lying down.
идти спать can sound a bit more casual, like “Alright, I’m going to bed now.”

Why is it поздно and not позже?

Поздно is an adverb meaning “late”.
- поздно ложусь спать = “I go to bed late.”
Позже means “later” (comparative of поздно).
- ложусь спать позже (чем обычно) = “I go to bed later (than usual).”

In this sentence, we’re not directly comparing to something; we just say the bedtime is late in general, so поздно is correct.

Why does the sentence use мне кажется with мне in the dative case?

Мне кажется literally means “it seems to me”.

Кажется is 3rd person singular: “it seems”.
Мне is dative of я: “to me”.

Russian often uses the dative case for the “experiencer” of feelings or perceptions:

мне холодно – “I’m cold” (literally “to me [it is] cold”).
мне нравится – “I like it” (literally “it is pleasing to me”).
мне кажется – “it seems to me / I feel like”.

So grammatically, кажется is impersonal and мне just marks who is experiencing that impression.

If мне is “to me”, why doesn’t Russian say something like я кажусь?

Кажется in мне кажется is not reflexive “I seem (to someone)”. It works like an impersonal verb:

кажется = “it seems (that)…”.
мне кажется = “it seems to me (that) …”.

Saying я кажусь would mean “I seem / I appear (to others)”, e.g.:

Я кажусь странным. – “I seem strange.”

That’s a completely different meaning from “I feel like / it seems to me that …”.

Is что after мне кажется mandatory? Could I say мне кажется, я зря не дал себе поспать больше?

Both forms are possible:

Мне кажется, что я зря не дал себе поспать больше. (with что)
Мне кажется, я зря не дал себе поспать больше. (without что)

Including что is slightly more explicit/formal, but in speech it’s very common to drop что after verbs like думать, кажется, знать, понимать etc.

So your shorter version is natural and correct.

Why is дал in the past tense, while ложусь and кажется are in the present?

The time relationships are different:

Если я поздно ложусь спать – a general, present-time or habitual condition: “if/when I go to bed late”.
мне кажется – a present reaction: “it seems to me / I feel”.
я зря не дал себе поспать больше – refers to some decision in the past relative to that moment: “I didn’t let myself sleep more (earlier), and that was pointless / a mistake.”

So the pattern is:

Present general condition (ложусь)
Present feeling about it now (кажется)
Past specific action you are regretting (не дал себе поспать больше)

Russian is comfortable mixing these tenses as long as the implied times make sense.

What does зря add here, and where can it go in the sentence?

Зря roughly means:

“in vain, for nothing, needlessly, pointlessly, foolishly, I shouldn’t have”
Here it expresses regret: “I shouldn’t have / it was a bad idea that I didn’t let myself sleep more.”

Placement:

я зря не дал себе поспать больше
я не зря дал себе поспать больше (completely different meaning: “I was right to let myself sleep more.”)

It usually comes before the verb or directly after the subject. In this sentence, я зря не дал… is the natural position. Moving зря around too much can sound awkward or change the focus.

Why is it не дал себе поспать, with себе in the dative case?

Дать + someone (in dative) + infinitive means “let / allow someone to do something”:

дать ему спать – “to let him sleep”
дать себе отдохнуть – “to let oneself rest”

Here:

дал – “I gave / I let”
себе (dative of себя) – “to myself”
поспать – “to sleep (for a while)”

So дал себе поспать = “I let myself sleep (for some time)”.
With не: не дал себе поспать = “I didn’t let myself sleep” / “I didn’t allow myself more sleep.”

Why is it поспать and not just спать?

Спать vs поспать:

спать (imperfective) – just the state/action “to sleep”, without boundary or result.
поспать (perfective) – “to have some sleep / to sleep for a while (and be done)”, focusing on the completed chunk of sleep as a result.

In дал себе поспать, the idea is “to let yourself get some sleep” as a finished amount. Using perfective поспать sounds natural because you’re talking about the result (having had more sleep), not the ongoing process.

Could the sentence be я зря не дал себе спать больше instead? What’s the difference?

You could say я зря не дал себе спать больше, but it sounds less idiomatic and slightly off.

Nuance:

дал себе поспать больше – “I let myself get more sleep / “have a longer sleep” (a completed period).
дал себе спать больше – “I let myself sleep more” as an ongoing state; it feels less like a natural Russian chunk here.

In contexts like permitting a state to continue, дать спать can work, but with себе + regret about not sleeping longer, поспать is the usual choice.

Why is the word order я зря не дал себе поспать больше and not, for example, я не дал себе зря поспать больше?

Word order in Russian is relatively flexible, but not all orders sound natural.

я зря не дал себе поспать больше – neutral, natural: “I foolishly didn’t let myself sleep more.”
я не дал себе зря поспать больше – sounds like “I did not let myself sleep more for nothing,” i.e. your sleep would have been pointless. That’s not what is meant.

Зря is qualifying the decision not to give yourself more sleep, not the act of sleeping. So it belongs close to не дал, not to поспать.

Is the second я necessary in что я зря не дал себе поспать больше? Could you drop it?

Yes, you can drop it:

Full: …мне кажется, что я зря не дал себе поспать больше.
Shorter: …мне кажется, зря не дал себе поспать больше.

In spoken Russian, omitting a repeated я like this is common when it’s clear who the subject is. The meaning stays the same; the shorter version is just a bit more colloquial.

Can you explain the overall structure of the sentence in simple pieces?

Broken down:

Если я поздно ложусь спать,
– “If/When I go to bed late,”
(condition / situation)
мне кажется,
– “it seems to me / I feel like”
(introduces your evaluation / feeling)
(что) я зря не дал себе поспать больше.
– “(that) I pointlessly / foolishly didn’t let myself sleep more.”
(what exactly seems to you; your regret)

Grammar-wise:

если-clause (present habitual)
main clause with dative experiencer (мне кажется)
subordinate clause with past action and regret (я зря не дал себе поспать больше).