Learn Hardcore Japanese: kare ha gogo ni naru to sukosi nemui. - He gets a little sleepy in the afternoon.

Here と is the conditional meaning when/whenever/once.
Pattern: plain present verb + と
- result. It expresses a natural, automatic result.
The result clause is typically non-volitional (not a command/request).
Example: 雨が降ると 寒くなる。= When it rains, it gets cold.

Why is に used after 午後? Is it the time particle?

In 午後になる, the に is part of the change-of-state pattern X になる (“become X”)—it marks the resulting state.
So 午後になる literally means “it becomes afternoon.”
With other verbs に can mark time (e.g., 三時に行く), but with なる here it’s the “to” of “become.” Examples: 大人になる, 三時になる.

Could I say 午後は instead of 午後になると?

Yes: 彼は午後は少し眠い。 = “As for afternoons, he’s a little sleepy.”
Nuance: 午後は states a general tendency/contrast; 午後になると highlights the change in time that triggers the state.
Both are natural for habitual statements.

Why is it 眠い and not 眠くなる?

眠い describes the state after the condition is met: “Once it’s afternoon, he is a little sleepy.”
眠くなる emphasizes the change: 午後になると少し眠くなる = “He starts to get a little sleepy in the afternoon.”
Choose based on whether you want to stress the state or the becoming.

What’s the rule for になる vs くなる?

Noun / na-adjective + になる: 医者になる, 元気になる.
i-adjective + くなる: 暑くなる, 眠くなる.
Here: 午後になる (午後 = noun). The alternative would be 眠くなる (眠い = i-adjective).

How does と compare with たら and とき here?

X と Y: whenever/once X happens, Y happens as a natural result; good for habits/facts; result is non-volitional.
X たら Y: when/if/after X, then Y; fine for one-time future/past; result can be volitional. Example: 午後になったら コーヒーを飲む。
X とき Y: “when/at the time (that) X.” Example: 学生のとき よく昼寝した。 For afternoons, you wouldn’t say “午後のとき”; use 午後は or 午後になると instead.

Is this sentence talking about a habit?

Yes. Plain present often states general tendencies. It means he typically gets a bit sleepy in the afternoon.

Can I omit 彼は?

Yes. 午後になると少し眠い。 is natural when the subject is obvious from context (often “I”).
Japanese frequently drops pronouns when they’re understood.

Is it okay to state that someone else is 眠い?

It’s fine when reporting a known tendency or something the person says about themselves.
If you’re just inferring from appearance, use 眠そうだ (seems sleepy) or 眠がっている (is acting sleepy).

How do I make the sentence polite?

Make the final predicate polite: 彼は午後になると少し眠いです。
The clause before と typically stays in plain form. A very formal announcement style is 午後になりますと, but that’s not needed in everyday speech.

Where does 少し go? Can I use ちょっと?

Put adverbs like 少し before the adjective: 少し眠い. You can’t say ✗ 眠い少し.
ちょっと is a casual alternative: 午後になるとちょっと眠い。 It can sound softer/more colloquial.

Who is the subject of なる in 午後になる?

There’s no explicit subject; it’s like English “it becomes afternoon.” Japanese often omits dummy subjects for time/weather: 三時になる, 遅くなる, 雨が降る.

Are the spaces between words normal in Japanese?

No. Standard Japanese is written without spaces. Natural orthography: 彼は午後になると少し眠い。
The spaced version is a learner aid.

Can I use 昼 instead of 午後?

昼 is “midday/daytime,” often around lunchtime; 午後 is “afternoon” (p.m., after noon).
If you mean around noon, say 昼ごろになると少し眠い. For the whole afternoon period, 午後 is more accurate.