Когда мне не спится, я пишу короткую запись в дневник о том, за что я благодарен.

The phrase мне не спится is an example of an impersonal construction in Russian.
Literally, you could think of it as:

• мне – to me (dative case)
• спится – is sleeping / feels like sleeping (an impersonal verb form)

So мне не спится is something like “it doesn’t sleep to me” → idiomatically: “I can’t sleep / I don’t feel like sleeping.”

Why dative (мне)?
In impersonal sentences about states and feelings, Russian often uses the dative to mark the experiencer:

• мне холодно – I am cold (literally: “cold is to me”)
• мне грустно – I am sad
• ему не работается – he doesn’t feel like working / it’s hard for him to work

So мне не спится fits this pattern: “it doesn’t sleep (itself) to me.”

All three talk about not sleeping, but with different nuances:

1. мне не спится

   • Focuses on the state/feeling: “I just can’t sleep / I’m not in a sleeping mood.”
   • Very natural when you’re lying in bed but can’t fall asleep; it often implies restlessness, insomnia, or simply not being able to drift off.

2. я не сплю

   • Literally: “I am not sleeping.”
   • Neutral, factual. You might say this if someone thinks you are asleep, and you correct them.
   • Doesn’t automatically imply that you want to sleep but can’t.

3. я не могу уснуть

   • Literally: “I cannot fall asleep.”
   • Explicitly about the inability to fall asleep (transition from awake to asleep).
   • Stronger and more objective than мне не спится; it sounds more like a clear problem: “I can’t get to sleep.”

In your sentence, Когда мне не спится… has a natural, slightly emotional feel: Whenever I just can’t sleep / whenever I can’t get to sleep…

Спится comes from the verb спать (to sleep), but here we have a special impersonal reflexive form:

• Base verb: спать
• Impersonal form: спится

The -ся ending can have several functions in Russian; one of them is to form impersonal verbs describing how some activity “goes” or “happens” (often with a feeling or mood):

• работать → работается – “to work” → “(it) works / one feels like working”
  
  • мне хорошо работается – I work well / it goes well for me.
• петь → поётся – “to sing” → “(it) sings / one feels like singing”
  
  • ей сегодня не поётся – she doesn’t feel like singing today.

Similarly:

• спать → спится
  
  • мне хорошо спится – I sleep well.
  • мне не спится – I can’t sleep / I don’t feel like sleeping.

So -ся here helps make a verb that describes an impersonal state: how sleeping “goes” for someone.

Короткую запись is in the accusative case, because it is the direct object of the verb пишу (“I write”).

• я пишу что? – короткую запись (what do I write? a short entry)

The noun запись is feminine. The adjective короткий has to agree with it in gender, number, and case:

• Nominative (dictionary form):
  
  • короткая запись – a short entry (as subject: эта короткая запись интересная)
• Accusative feminine (after a verb, as direct object):
  
  • короткую запись – I write a short entry (я пишу короткую запись)

So я пишу короткая запись would be ungrammatical because the adjective and noun would be in the nominative, not agreeing with their syntactic role as an object.

Both писать запись and делать запись can be used, but they have slightly different feels and collocations.

• писать запись (в дневник) – literally “to write an entry (in a diary).”

  • Very natural for diary/journal contexts, because you are physically writing or typing text.

• делать запись – literally “to make a note/entry.”

  • Very common for more “technical” or neutral notes:
    
    • делать запись в блокнот – to make a note in a notebook
    • делать запись разговора – to record a conversation (audio/video)

In a personal diary context, я пишу короткую запись в дневник is absolutely natural and idiomatic.
You could also say:

• я делаю короткую запись в дневник – also correct, just a bit more neutral; can sound slightly more “procedural.”

The difference comes from the case after в:

• в + accusative – direction, movement “into” something
• в + prepositional – location, “in/inside” something

In your sentence:

• запись в дневник (accusative) – an entry into the diary (the destination of the writing)
  
  • пишу запись в дневник – I write an entry into the diary.

If you said:

• запись в дневнике (prepositional) – an entry in the diary, describing where it is located:
  
  • запись в дневнике очень личная – the entry in the diary is very personal.

So:

• Action of writing → в дневник (where you are putting it, destination).
• Description of where it exists → в дневнике (its location).

You could say запись в журнал, but it changes the meaning:

• дневник – diary, journal (personal; often private)
• журнал – magazine, or a more formal “logbook/journal” (e.g. class record book, scientific journal)

Запись в дневник strongly suggests a personal diary entry: thoughts, feelings, gratitude, etc.
Запись в журнал could be:

• An entry in a logbook (e.g. ship’s log, work log)
• A magazine article (“an article in a journal/magazine”), though for articles you’d usually use статья в журнале.

For the idea of a personal gratitude journal, дневник is the most natural choice.

The phrase о том, за что я благодарен breaks down like this:

• о том – about that (about the thing)
• за что – for what (for which things)
• я благодарен – I am grateful

О том is a demonstrative pronoun (то in the prepositional case: о том). It refers to what you are writing about in general: about the fact / about the things…

За что introduces a relative clause specifying what exactly you are grateful for:

• о том, за что я благодарен – about what I am grateful for
  Literally more like: “about that for which I am grateful.”

Russian frequently uses this structure о том, что… / о том, как… / о том, почему… etc.:

• я думаю о том, что будет завтра – I think about what will happen tomorrow.
• он рассказал о том, как он путешествовал – he talked about how he traveled.

In your sentence, о том, за что я благодарен is a single unit: “about what I am grateful for.”

With gratitude, Russian normally uses за + accusative to express “for (something)”:

• спасибо за помощь – thank you for the help.
• я благодарен тебе за поддержку – I am grateful to you for your support.
• мы благодарны за внимание – we are grateful for your attention.

So the pattern is:

• быть благодарным (кому?) за (что?)
  
  • кому? – dative (to whom you are grateful)
  • за что? – accusative (for what you are grateful)

In your sentence, the “to whom” part is omitted (it could be “to life,” “to God,” “to people,” etc.), but за что is still used for “for what.”

Для usually means “for (the purpose/benefit of)”:

• лекарство для детей – medicine for children.

О usually means “about”:

• я думаю о тебе – I think about you.

So за is the correct preposition to express thanks/gratitude for something in Russian.

Благодарен is the short-form adjective, used mainly as a predicate (after “to be”), to describe a temporary state or fact:

• я благодарен – I am grateful.
• он был благодарен друзьям – he was grateful to his friends.

Благодарный is the full-form adjective, more like “grateful” as a characteristic/quality:

• благодарный человек – a grateful person (as a trait, more permanent or descriptive).

In modern Russian:

• To say “I am grateful (right now / in this situation)”, you almost always use the short form:
  
  • я благодарен (за что?)

Using я благодарный as a standalone statement sounds odd or stylistically marked, almost like “I am a grateful kind of person” (character trait), and even then people usually add a noun:

• я благодарный ученик – I am a grateful student.

So я благодарен is the natural, idiomatic way to say “I am grateful” in this context.

The only word that changes is благодарен, because it agrees with the subject in gender and number.

1. Female speaker (я, feminine):

   • Когда мне не спится, я пишу короткую запись в дневник о том, за что я благодарна.
   • благодарна – short-form feminine.

2. Group speakers (мы):

   • Когда нам не спится, мы пишем короткую запись в дневник о том, за что мы благодарны.
   • нам instead of мне (dative plural),
   • пишем instead of пишу,
   • благодарны – short-form plural.

3. Male speaker (original sentence):

   • Когда мне не спится, я пишу короткую запись в дневник о том, за что я благодарен.

Short-form adjectives like благодарен / благодарна / благодарны must match the logical subject.

Yes, я пишу в дневник короткую запись is grammatically correct. Word order in Russian is relatively flexible, and both are possible:

• я пишу короткую запись в дневник
• я пишу в дневник короткую запись

Nuance:

• The original order (короткую запись в дневник) flows very naturally: you first say what you write (короткую запись) and then where (в дневник).
• я пишу в дневник короткую запись slightly emphasizes the destination в дневник first (“I write into the diary a short entry”), and then clarifies what kind of thing it is.

In everyday speech, the original version is probably more common, but both are acceptable and clear.

Пишу is the imperfective aspect of писать. In this sentence, you’re describing a repeated, habitual action:

• Когда мне не спится, я пишу… – Whenever I can’t sleep, I (usually) write…

Imperfective aspect is used for:

• Habits / repeated actions:
  
  • когда мне скучно, я читаю – when I’m bored, I read.
• Ongoing processes:
  
  • я сейчас пишу письмо – I am writing a letter now.

A perfective form like напишу would focus on a single, completed action:

• когда мне не спится, я напишу короткую запись…
  This would sound like a one-time decision in the future: “When I can’t sleep (on some future occasion), I’ll write a short entry…”

Since your sentence describes a general habit, the imperfective пишу is the natural choice.