A única desvantagem de estudar de noite é passar o dia seguinte com sono.

In Portuguese, when you use único/única in the sense of “the only”, you almost always need the definite article o/a:

• A única desvantagem = the only disadvantage
• O único problema = the only problem

Without the article, única desvantagem would sound incomplete or like part of a longer phrase (e.g. after a verb: “é única desvantagem que...”), and even then, native speakers still tend to keep the article.

So here, A is just the normal definite article for a feminine singular noun (desvantagem), and it’s required to sound natural.

After vantagem (advantage) and desvantagem (disadvantage), the normal preposition in Portuguese is de, not em:

• A vantagem de acordar cedo…
• A desvantagem de morar longe…

So:

• A única desvantagem de estudar de noite…

Using em (desvantagem em estudar) is not standard; it sounds unnatural to native speakers. The pattern is essentially fixed: (des)vantagem de + verb / noun.

In Portuguese, after a preposition like de, you must use the infinitive form of the verb:

• de estudar
• sem comer
• para dormir

Unlike English, Portuguese does not use the gerund (-ing form) after prepositions:

• English: advantage of studying at night
• Portuguese: vantagem de estudar à noite (not de estudando à noite)

So de estudar is the only natural option here. The gerund estudando would be wrong after de in this structure.

Both are correct and common in European Portuguese:

• de noite
• à noite

In practice, they almost always mean the same: “at night / in the evening”.

Slight nuance (often subtle and not always felt):

• à noite can feel a bit more “neutral” or habitual:
  
  • Gosto de estudar à noite. – I like studying at night.
• de noite can sometimes feel a bit more descriptive of the time of day (like “during the night”), but it’s also widely used just like à noite:
  
  • Trabalho de noite. – I work nights / at night.

In your sentence, you could say either:

• …desvantagem de estudar de noite…
• …desvantagem de estudar à noite…

Both are fine in European Portuguese.

Literally:

• passar o dia seguinte com sono
  = to spend the following day feeling sleepy

In Portuguese, passar + an expression of time is a very common way to say “spend (time)”:

• passar o dia a trabalhar – to spend the day working
• passar a tarde em casa – to spend the afternoon at home
• passar a noite a estudar – to spend the night studying

So here:

• passar o dia seguinte com sono
  means you are sleepy for most or all of the next day, not just at a single moment.

If you said estar com sono no dia seguinte, it would describe a state at some point on the next day. Passar o dia seguinte com sono emphasizes that the whole day is affected.

Because in this structure, o dia seguinte is the direct object of the verb passar:

• passar o dia – to spend the day
• passar o fim de semana – to spend the weekend
• passar o dia seguinte – to spend the following day

If you used no dia seguinte (em + o), that would be an adverbial phrase of time, not the object:

• Estive com sono no dia seguinte. – I was sleepy the next day.

So:

• passar o dia seguinte com sono = spend the next day feeling sleepy.
• estar com sono no dia seguinte = be sleepy the next day (at some time).

The sentence chooses the “spend the day” structure, so it needs o dia seguinte as a direct object.

Portuguese has a very common fixed expression:

• estar com sono – literally to be with sleep, meaning “to feel sleepy”.

From this, you also get:

• ficar com sono – to become sleepy
• andar com sono – to go around feeling sleepy
• passar o dia com sono – to spend the day feeling sleepy

You could say:

• sonolento – drowsy, sleepy
  
  • passar o dia seguinte sonolento – also grammatically correct
• cansado – tired (more about general fatigue, not just sleepy)

But in everyday speech, com sono is far more natural and idiomatic than sonolento in this kind of sentence.

Sono in this sense (sleepiness) is uncountable in Portuguese, like “sleep” in English in I need sleep.

So you say:

• Tenho sono. – I’m sleepy.
• Estou com sono. – I’m (feeling) sleepy.
• Ando com muito sono. – I’ve been very sleepy.

You don’t use sonos (plural) for normal “sleepiness”. Sonos can appear in specific idiomatic or technical contexts, but not in everyday expressions like this.

So com sono is the correct and natural form here.

In Portuguese, adjectives usually come after the noun, especially in neutral, literal descriptions:

• dia seguinte – the following day
• próximo ano – next year
• último mês – last month

Putting the adjective before (seguinte dia) is ungrammatical here. Seguinte in this meaning (“following”) always goes after the noun:

• o dia seguinte – the next day
• na página seguinte – on the next page

So the order dia seguinte is simply the normal grammar rule: noun + adjective.

Yes, you could say:

• A única desvantagem de estudarmos de noite…

This is the infinitivo pessoal (personal infinitive), which agrees with an understood subject (nós here).

Difference:

• de estudar de noite – general idea of “studying at night” (anyone in general).
• de estudarmos de noite – more clearly “our studying at night” (we).

Both are grammatically correct. The original sentence is more general or impersonal; estudarmos would make it sound more like “for us, the only disadvantage of studying at night is…”.