Learn Hardcore Russian: Когда я соблюдаю режим сна, сонливость днём почти исчезает. - When I follow a sleep schedule, daytime sleepiness almost disappears.

In real usage, режим сна is exactly how Russians say “sleep schedule / sleep routine” in a neutral, natural way, especially in medical, health, or everyday contexts:

соблюдать режим сна – to follow/keep a sleep schedule
нарушать режим сна – to disrupt one’s sleep schedule

You might also see режим сна и бодрствования (“sleep–wake schedule”), but режим сна alone is perfectly normal and common.

Why is it режим сна and not режим сон? What case is сна?

Сна is the genitive singular of сон (“sleep”).

The pattern [режим + Genitive] is common: the genitive shows what the “regimen” is about:

режим сна – sleep regimen / sleep schedule
режим питания – eating/diet regimen
режим дня – daily routine (literally “regimen of the day”)

So сон (“sleep”) changes to сна (“of sleep”) to fit this pattern.

What exactly does соблюдать mean here, and why not use следовать or something else?

Соблюдать means “to observe / to comply with / to stick to” (rules, a schedule, a regime).

Common collocations:

соблюдать режим сна – to keep/observe a sleep schedule
соблюдать диету – to stick to a diet
соблюдать правила – to follow rules

You could say следовать режиму сна, but that is less idiomatic. The strong, standard collocation is соблюдать режим (режим сна, режим дня, режим питания, etc.), so соблюдаю режим сна sounds most natural.

What aspect and tense is соблюдаю, and why is that form used here?

Соблюдаю is present tense, imperfective aspect of соблюдать.

Imperfective is used for repeated, habitual actions or ongoing processes.
The sentence describes what usually happens whenever the speaker follows their sleep schedule.

So Когда я соблюдаю режим сна means “When(ever) I follow my sleep schedule (as a regular or repeated thing),” which matches the use of the imperfective present.

What about исчезает – what aspect and nuance does it have here?

Исчезает is present tense, imperfective of исчезать (“to disappear”).

Here it describes what regularly happens as a result: whenever the condition is met, the sleepiness almost disappears. The imperfective present is standard for general cause–effect statements:

Когда я пью кофе, сонливость исчезает.
“When I drink coffee, the sleepiness disappears.”

If you used the perfective исчезнуть (e.g. исчезнет), it would sound more like a one-time future result, not a general habit.

Why is there a comma after Когда я соблюдаю режим сна?

Когда introduces a subordinate clause of time. In Russian, a subordinate clause is normally separated from the main clause by a comma.

Когда я соблюдаю режим сна, – subordinate clause
сонливость днём почти исчезает. – main clause

So the comma is required. Writing it without a comma (Когда я соблюдаю режим сна сонливость днём почти исчезает) would be considered a punctuation error.

Could I leave out я and say Когда соблюдаю режим сна, сонливость днём почти исчезает?

Yes, Когда соблюдаю режим сна, … is possible and understandable, and Russians sometimes omit subject pronouns when the subject is obvious from the verb ending.

However:

In a neutral, written style, Когда я соблюдаю… is more typical and slightly clearer.
Omitting я feels a bit more colloquial or “diary-like”: “When I (do) keep a sleep schedule, …”

So the original version with я is the most standard and learner‑friendly.

What is днём exactly? Why isn’t it днём in the nominative or something like день?

Днём is the instrumental singular of день (“day”), used adverbially to mean “in the daytime / during the day.”

Russian often uses the instrumental to make adverbial expressions of time:

утром – in the morning
днём – in the daytime
вечером – in the evening
ночью – at night

So сонливость днём means “sleepiness (that I feel) during the day” or “daytime sleepiness.” It’s not the subject; it’s a time modifier.

Can днём go in a different position? For example, Сонливость почти исчезает днём?

Yes, Russian word order is flexible. All of these are grammatically correct:

Сонливость днём почти исчезает.
Сонливость почти исчезает днём.
Днём сонливость почти исчезает.

The differences are mostly about focus:

Сонливость днём почти исчезает. – neutral; “Daytime sleepiness almost disappears.”
Сонливость почти исчезает днём. – slightly more focus on when it almost disappears.
Днём сонливость почти исчезает. – strong emphasis on daytime as the context.

The original order is natural and neutral.

How should I pronounce днём, and why is there ё instead of е?

Днём is pronounced approximately [dnyom]:

дн- is like “dn” in “midnight” but within one syllable
-ём is like English “yom” (as in Yom Kippur), with stressed yo

The letter ё always indicates stressed “yo” ([jo]). In everyday writing, Russians often replace ё with е, so you may see днём written as днем, but the correct pronunciation is still [днём].

What does почти add here? Could the sentence work without it?

Почти means “almost / nearly”.

With почти: сонливость днём почти исчезает = “daytime sleepiness almost disappears” (it’s reduced a lot, but not 100%).
Without почти: сонливость днём исчезает = “daytime sleepiness disappears” (the idea is that it goes away completely).

So почти softens the claim, making it more realistic: it’s not fully gone, but greatly reduced.

Could почти go somewhere else, like почти сонливость днём исчезает?

In this sentence, почти should modify the verb исчезает, not the noun сонливость.

Natural placements:

сонливость днём почти исчезает
сонливость днём исчезает почти полностью (“disappears almost completely”)

Почти сонливость днём исчезает is wrong or very unnatural: it would sound like you are saying “almost the sleepiness disappears”, which is not how Russian expresses this idea.

What’s the difference between когда and если in this kind of sentence? Could I say Если я соблюдаю режим сна…?

Both когда and если are possible here, but they have different nuances:

Когда я соблюдаю режим сна, сонливость днём почти исчезает.
Focus on time/whenever – whenever I keep a sleep schedule, this is the typical result. Very natural for describing a regular pattern.
Если я соблюдаю режим сна, сонливость днём почти исчезает.
Focus on condition/if – if I happen to follow my sleep schedule, then this happens. Slightly more conditional or hypothetical.

In this context (describing a regular personal experience), когда is the more neutral and common choice, but если would not be wrong.

Is сонливость the only way to say “sleepiness”? How is it different from сонность or мне хочется спать?

Сонливость is a noun meaning “sleepiness, drowsiness”, often used in more formal, medical, or descriptive language:

повышенная сонливость – increased sleepiness
дневная сонливость – daytime sleepiness

Other options:

сонность – similar meaning, also “sleepiness/drowsiness,” but sounds a bit more abstract or technical.
мне хочется спать / я хочу спать – “I feel sleepy / I want to sleep,” i.e. a personal statement about how you feel at the moment.

In this sentence, сонливость днём works well because we are talking about a general symptom/state, not about one specific momentary feeling.

Why is сонливость feminine, and how do we know which gender to use?

Сонливость ends in -ость, and almost all nouns ending in -ость are feminine in Russian:

скорость (speed) – feminine
молодость (youth) – feminine
реальность (reality) – feminine

So сонливость is feminine, and the verb form исчезает does not change with gender (verbs in the present tense are the same for all genders in 3rd person singular). The gender matters more for adjectives and past tense:

сильная сонливость (strong sleepiness) – feminine adjective
сонливость днём почти исчезла (past tense, feminine form)