Why you feel tired after watching porn?

Right after masturbation the chemical Prolactin is released. This chemical makes people feel tired, sleepy and exhausted.

Watching many porn images puts a high load on the nervous system and this may result in nervous system fatigue. 

When a person watches porn excess amounts of Dopamine and Sertonin are produced. The excess hormones over works the brain and body thus result in feelings of exhaustion when a person stops. (See How to increase serotonin naturally)

Adrenaline is formed of Dopamine. The excess dopamine results in the formation of adrenaline for a prolonged period of time. Thus puts the body and the brain in the fight or flight response for an extended period of time.

Most people try to hide from others , especially family members, when they watch porn. The fact that they feel anxious about getting caught can result in prolonged feelings of anxiety as long as they are watching porn. (See The psychological benefits of breaking porn addiction)

Dopamine is the chemical responsible for the go-get-it behavior.  When a person gets used to watching porn his brain gets used to a high amount of Dopamine. When the person stops the natural dopamine produced by the brain fails to motivate him and so he might feel tired and unmotivated.

Many people experience bad emotions right after watching porn which include shame, guilt and low self esteem. Those feelings combined can make a person feel tired, sad and unmotivated. 

Staying in front of a computer screen for long hours can exhaust the vision and overload the parts of the brain responsible for visual processing.

As a person's brain gets used to high amount of dopamine the naturally produced dopamine by the body fails to do its normal functions. Those functions include staying alert, staying focused and getting motivated. If those functions can't be carried on properly a person might feel tired.