Nukkuminen on helpompaa, kun makuuhuone on viileä.

Nukkuminen is a -minen noun (a verbal noun). Finnish often uses this form to talk about an activity in a general, abstract way—basically “sleeping” as a concept, not “(someone) sleeps.”

• nukkua = to sleep (verb)
• nukkuminen = sleeping (the act/activity, noun)

So the sentence is built like a normal noun sentence: Nukkuminen on … = “Sleeping is …”

Both can exist, but they mean slightly different things/fit different structures.

In Nukkuminen on helpompaa, helpompaa is partitive singular of the comparative adjective helpompi. With abstract subjects like nukkuminen, Finnish very often uses the partitive in the predicate to express a general/partial quality:

• Nukkuminen on helpompaa ≈ “Sleeping is easier (in general / to some extent).”

Nukkuminen on helpompi (nominative) sounds odd to most speakers here because it treats “sleeping” more like a countable “item” being identified as “the easier one,” which doesn’t match the idea.

A very natural alternative structure is:

• On helpompi nukkua, kun… = “It’s easier to sleep when…”

Steps: 1) Base adjective: helppo = easy
2) Comparative: helpompi = easier
3) Partitive singular of helpompi: helpompaa

So:

• helppo → helpompi → helpompaa

In this sentence, kun is best understood as “when” in the sense of “whenever / in the situation that.” It sets a condition-like time/situation:

• …kun makuuhuone on viileä = “…when the bedroom is cool.”

Depending on context, kun can sometimes be close to “since/because,” but here it’s mainly a condition/situation: sleeping becomes easier in the circumstances where the bedroom is cool.

(If you wanted to emphasize a clear reason, Finnish often uses koska = “because,” but it changes the feel.)

Finnish normally uses a comma before a subordinate clause introduced by kun (and many other conjunctions), similar to English:

• Main clause: Nukkuminen on helpompaa
• Subordinate clause: kun makuuhuone on viileä

So the comma marks the boundary between the two clauses.

Because makuuhuone is a concrete, countable noun and the sentence is a straightforward “X is Y” description.

• makuuhuone = bedroom (singular, nominative)
• Predicate adjective agrees in case/number in this type of sentence: viileä (nominative singular)

So:

• Makuuhuone on viileä. = “The bedroom is cool.”

Viileää (partitive) would suggest something like “somewhat cool” / “coolish” in certain contexts, but the default, neutral description is viileä.

Finnish has no articles, so makuuhuone can mean “a bedroom” or “the bedroom” depending on context.

In a general statement like this, English often chooses “the bedroom” (“…when the bedroom is cool”) even though Finnish is just stating the condition generically.

Yes. Finnish commonly uses the present tense for general truths, habits, and general recommendations:

• Nukkuminen on helpompaa… = “Sleeping is easier…” (general statement)

It’s not “right now sleeping is easier,” but “in general, sleeping tends to be easier under these conditions.”

Yes, that’s completely correct and common. Finnish allows flexible word order, and you can front the kun-clause for emphasis or flow:

• Kun makuuhuone on viileä, nukkuminen on helpompaa. = “When the bedroom is cool, sleeping is easier.”

The comma is still used.

A very natural alternative is the impersonal construction:

• On helpompi nukkua, kun makuuhuone on viileä.
  Literally: “It is easier to sleep when the bedroom is cool.”

Another common option uses an adessive location phrase:

• Viileässä makuuhuoneessa on helpompi nukkua. = “In a cool bedroom it’s easier to sleep.”

All of these are natural; the original Nukkuminen on helpompaa… is also perfectly fine and slightly more abstract/formal.