Can I Pray Sitting Down If I'm Tired?
Question

When is it permissible for a sick person to pray sitting down, because he may be able to stand but with great difficulty?
Praise be to Allah.Is standing a pillar of prayer?
In the answer to question no 50684  we have stated that standing is a pillar or essential part of the obligatory prayers, so the prayer of one who prays sitting when he is able to stand is not valid. But like all other obligatory duties, this may be waived in the case of inability to do it. 
An-Nawawi (may Allah have mercy on him) said in Al-Majmu` (4/201): 
“The Ummah unanimously agreed that whoever is unable to stand in an obligatory prayer may pray sitting down, and he does not have to repeat the prayer. Our companions said: his reward will not be less than the reward for offering the prayer standing up, because he is excused. It was proven in Sahih Al-Bukhari that the Messenger of Allah (peace and blessings of Allah be upon him) said: “If a person is sick or is travelling, the reward of what he used to do when he was healthy and not travelling with be written for him.””  
Excuses which allow praying sitting down
The guidelines for the excuse which allows one not to stand and to offer obligatory prayers sitting down , are as follows: 

That he be unable to stand
That it will make his sickness worse
That it will delay his recovery
That it should cause great difficulty of such a scale that he will be unable to focus properly. If the difficulty caused is less than that, then it is not permissible for him to sit.

Al-Bukhari (1117) narrated that `Imran ibn Husayn (may Allah be pleased with him) said: I had haemorrhoids and I asked the Prophet (peace and blessings of Allah be upon him) about praying. He said: “Pray standing, and if you cannot, then whilst sitting, and if you cannot, then whilst lying on your side.” 
Al-Hafidh (may Allah have mercy on him) said: “The words “If you cannot” were quoted as evidence by those who say that a sick person should not start to sit whilst praying until after he becomes unable to stand. This was narrated by `Iyad from Ash-Shafi`i. It was narrated from Malik, Ahmad and Is-Haq that it is not essential that a person be unable to stand, rather if it is very difficult for him to stand (then he may sit). 
The well-known view among the Shafi`is is that what is meant by not being able is that it is very difficult to stand, or there is the fear that the sickness may be made worse or it may lead to the death of the patient. Slight difficulty is not sufficient (as an excuse ). Another example of great hardship is dizziness in the case of a passenger on a boat who fears that he may drown if he prays standing in the boat. 
The majority also quote as evidence the hadith of Ibn ‘Abbas which was narrated by At-Tabarani (may Allah have mercy on him): “He should pray standing, but if he encounters difficulty then sitting, and if he encounters difficulty, then lying down.”” (Fat-h Al-Bari) 
The Hadith of Ibn ‘Abbas (may Allah be pleased with him) which was mentioned by Al-Hafidh was also quoted by Al-Haythami in Majma` Al-Zawa’id, 2897, where he says: 
“This was narrated by At-Tabarani in Al-Awsat. He said: It was not narrated from Ibn Jurayj except by Halas ibn Muhammad Al-Dab’i. I (Al-Haythami) say: I cannot find anyone who wrote his biography, but the rest of the men in its chain of narrators are trustworthy.”
Ibn Qudamah (may Allah have mercy on him) said in Al-Mughni, 1/443: 
“If he is able to stand (he should do so), otherwise if he fears that his sickness will become worse or his recovery will be slowed or it is extremely difficult for him to stand, then he may pray sitting down. Something similar was said by Malik and Ishaq… because Allah says (interpretation of the meaning): 
“and has not laid upon you in religion any hardship” [Al-Hajj 22:78] 
The obligation of standing in this case is a hardship. 
And the Prophet (peace and blessings of Allah be upon him) prayed sitting down when he was wounded in his right cheek; it seems that he was not completely unable to stand, but because it was difficult for him to stand, he was excused.”
An-Nawawi (may Allah have mercy on him) said in Al-Majmu`, 4/201:
“Our companions said: Being unable does not necessarily mean that one cannot stand and slight difficulty is not enough reason for not standing; rather what counts is obvious difficulty. If a person fears immense hardship or worsening of the sickness etc., or if a passenger in a boat fears that he may drown or become dizzy, he may pray sitting down, and he does not have to repeat the prayer. Imam Al-Haramayn said: What I think with regard to the guidelines on what constitutes an inability to stand is that standing would cause hardship that would prevent him from focusing properly, because proper focus is the purpose of prayer.”
The view favoured by Imam Al-Haramayn (may Allah have mercy on him) is the view which Shaykh Ibn `Uthaymin (may Allah have mercy on him) thought most likely to be correct. He said:  
“The guideline on what constitutes hardship is that which prevents one from focusing properly. Proper focus means having presence of mind and being at ease. If standing will cause intense anxiety which makes one ill at ease and makes him wish to reach the end of Al-Fatihah quickly so that he can bow, because it is too hard to stand, then he may pray sitting down.” (Ash-Sharh Al-Mumti’, 4/326.
For more details, please refer to the following answers: 142657 , 143720 , 226368 , and 115143 .
And Allah knows best.