Cuando no duermo siesta, me siento cansado por la tarde.

Both duermo siesta and duermo la siesta are possible in many parts of the Spanish‑speaking world.

• Without article (duermo siesta): very common with some “routine” activities, especially in informal speech. The noun behaves a bit like an uncountable/generic activity:

  • duermo siesta = I take naps / I nap
  • tomo café = I drink coffee
  • hago ejercicio = I exercise

• With article (duermo la siesta): also correct; in many regions this is actually more common. It treats la siesta as a specific daily event:

  • duermo la siesta = I sleep my (usual) nap / I take my nap

In this sentence, omitting the article just sounds a bit more “activity-like” and general. Meaning-wise, both are fine.

Both are correct, but they are not identical:

• estar cansado = to be tired (state)
• sentirse cansado = to feel tired (how you experience your state)

Me siento cansado emphasizes your subjective feeling of tiredness:
> Cuando no duermo siesta, me siento cansado por la tarde.
> When I don’t nap, I feel tired in the afternoon.

You could also say:

• … *estoy cansado por la tarde.* (I am tired)

This version is perfectly natural too; it just sounds slightly more neutral and less explicitly about your internal perception.

Sentir has two different patterns:

1. sentir + noun (non‑reflexive): to feel something

   • Siento frío. = I feel cold.
   • Siento dolor. = I feel pain.
     Here you don’t use me.

2. sentirse + adjective/adverb (reflexive): to feel (in a certain way)

   • Me siento cansado. = I feel tired.
   • Me siento bien. = I feel good.

Because cansado is an adjective describing how I feel, you need the reflexive form me siento, not siento cansado.

Spanish adjectives must agree in gender and number with the subject:

• If the speaker is male:
  
  • Me siento cansado.
• If the speaker is female:
  
  • Me siento cansada.

The sentence is written with a masculine speaker in mind. A woman would say:
> Cuando no duermo siesta, me siento *cansada por la tarde.*

This is a general/habitual statement: something that regularly happens. Spanish normally uses the present for this kind of general truth:

• Cuando no duermo siesta, me siento cansado por la tarde.
  = Whenever I don’t take a nap, I feel tired in the afternoon.

You could say:

• Cuando no duerma siesta, *me sentiré cansado por la tarde.*

But that sounds more like talking about a specific future situation, not a general habit. For generic habits, present + present is the natural choice.

Cuando can take indicative or subjunctive, depending on the meaning:

• Indicative (as in the sentence):
  Used for habitual, repeated, or known situations:

  • Cuando *no duermo siesta, me siento cansado.*
    (This is something that happens regularly.)

• Subjunctive:
  Used for future, uncertain, or not-yet-realized events:

  • Cuando *no duerma siesta mañana, me sentiré cansado.*
    (Talking about a future case that hasn’t happened yet.)

Here we’re describing a general pattern, so indicative is correct: cuando no duermo…

Yes. Both orders are fine:

• Cuando no duermo siesta, me siento cansado por la tarde.
• Me siento cansado por la tarde cuando no duermo siesta.

Placing the cuando-clause at the beginning slightly emphasizes the condition (“whenever this happens…”). Putting it later emphasizes more the result (“I feel tired when this happens”). Grammatically, both are correct and natural.

For broad times of day, Spanish usually prefers:

• por la mañana = in the morning
• por la tarde = in the afternoon
• por la noche = at night

So por la tarde is the standard way to say “in the afternoon” (in a general sense).

You do see en la tarde or en la mañana in some Latin American regions, but por la tarde is widely understood and considered the most neutral/standard for “in the afternoon” as a general time frame.

De la tarde is typically used with clock times, not by itself:

• A las tres de la tarde = at 3 in the afternoon.

In Latin America you will hear several variants, often depending on the country:

• Dormir la siesta

  • Cuando no duermo *la siesta, me siento cansado…*
    Very common and maybe the most widespread.

• Dormir siesta (without article)
  Natural in some areas; used like a generic activity.

• Echar la siesta / hacer la siesta
  More typical in Spain, but you may still encounter them.

Your original sentence is understandable in Latin America, but “no duermo la siesta” would sound especially natural in many countries.

Not really, because siesta specifically means “nap” (usually in the early afternoon).

• No duermo siesta is understood as “I don’t take a nap (at midday/afternoon)”, not “I don’t sleep at all.”

To say I don’t sleep at all, you’d say:

• No duermo nada.
• No duermo en toda la noche.

Both siesta and tarde are feminine nouns:

• la siesta
• la tarde

That’s why we use la in por la tarde and (optionally) in (la) siesta:

• no duermo (la) siesta
• por la tarde

It also matters for adjectives that might describe them:

• una siesta larga (long nap)
• una tarde calurosa (hot afternoon)

People will understand tomo siesta, but the most idiomatic verbs are:

• dormir la siesta
• hacer la siesta (more in Spain)
• echar la siesta (more in Spain)

In Latin America, dormir la siesta is usually your safest and most natural choice:

Cuando no duermo la siesta, me siento cansado por la tarde.