Allah (Subhanahu Wa Ta'ala) has given this physical body to His worshippers as a trust. It is compulsory to fulfill the rights of the body. Hadhrat Abdullah bin 'Amr (May Allah be well pleased with him) was continuously fasting and praying in the night. The Holy Prophet (Sallallahu alaihi wa sallam) advised him to be moderate in his worship and to take care of his health. He (Sallallahu alaihi wa sallam) told him:
Translation: On you are the rights of your body and the rights of your eyes. (Sahih Bukhari, Hadith No: 1974)
By continuously missing Sehri (Pre-dawn prayer), the body becomes weak, which may lead to physical distress when Allah (Subhanahu Wa Ta'ala) says in Surah Baqra-195
It is Masnoon and Mustahab (established by the Sunnah and commendable) to have Sehri. The Holy Prophet (Sallallahu alaihi wa sallam) has ordered us to eat Sehri:
Translation: It has been narrated on the authority of Hadhrat Anas bin Malik (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: Have Sehri for there are blessings in Sehri. (Jame' Tirmidhi, Vol. 1, Pg No. 150)
If any person does not have Sehri and puts oneself in distress with the intention of disciplining the Nafs (base self), then instead of remaining hungry that person should eat as much as will allows him/her to fast easily and stand up in prayer, so that there is no compromise in worship of Ramadhan (Ibadah) and to gain the reward and blessings of having Sehri.
Translation of Hadith: For the worship of night, take help from siesta and for fast, from Sehri. (Ibn Majah, Kitab Us Siyam)
It is given in Fatawa A'alamgiri, Vol. 5, Pg No. 336:
Translation: The worshipper's having Sehri and fasting is preferred in the Shariah and it is not preferred to leave Sehri.
Having Sehri late is commendable (Mustahab). The time for Sehri is the last part of the night, i.e. just before true dawn. Thus, Sehri should be taken before true dawn. Taking it so late that dawn is suspected is Makruh.
At the time of Sehri (pre-dawn meal), we should ask Allah Ta’ala for His forgiveness. Allah Ta’ala describes the qualities of pious people:
And used to (get up and) seek forgiveness in the later part of the night. Surah Dhaariyaat (51:18)
There is a Hadith about the time between Sahr and Salaat Ul Fajr:
Translation of Hadith: Hadhrat Anas Bin Malik (May Allah be well pleased with him) narrates on the authority of Hadhrat Zaid Bin Thabit (May Allah be well pleased with him), he said: We had Sehri along with the Holy Prophet (Sallallahu alaihi wa sallam), then got up for Salaat. I asked: How much time in between? He said: As much as it takes to recite 50 verses.
(Jame' Tirmidhi, Hadith No: 707)
For this reason, the Fuqaha (Islamic Law Experts) have clarified that spending so much time in Sehri that the beginning time of Fajr comes closer is Makruh. Nowadays, Sehri should be stopped at the maximum time written in calendars, etc. as per the research of the scholars.