You should pray as long as you have the energy to do so. If you feel tired, then lie down
Authentic hadithعن أنس رضي الله عنه قال: دخل النبي صلى الله عليه وسلم المسجد فإذا حبل ممدود بين الساريتين، فقال: «ما هذا الحبل؟» قالوا: هذا حبل لزينب، فإذا فَتَرَتْ تَعَلَّقَتْ به. فقال النبي صلى الله عليه وسلم : «حُلُّوهُ، لِيُصَلِّ أحدكم نشاطه فإذا فَتَرَ فليرقد».
Anas (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) came one day into the mosque and saw a rope stretched between two poles. He inquired: "What is this rope for?" He was told: "This is Zaynab's rope. When she feels tired (in voluntary prayer), she holds on to it for support." The Prophet (may Allah's peace and blessings be upon him) said: "Untie it. You should pray as long as you have the energy to do so. If you feel tired, then lie down."
Explanation
The Prophet (may Allah's peace and blessings be upon him) entered the mosque and saw a rope tied between two poles. He was surprised at that and asked why the rope was tied like that. The Companions (may Allah be pleased with him) answered that the rope belonged to Zaynab bint Jahsh (may Allah be pleased with her). She would pray lengthy voluntary prayers, and when she felt tired, she would continue to pray while holding on to the rope. He ordered them to remove the rope and instructed them to be moderate in worship without going to extremes and overburdening themselves so that they perform worship with energy and vitality.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).