Short answer
In the Hanafi school a period starts when blood reaches the outer part of the private area, and you go by when you knew. If you slept clean and woke to blood, your period counts from when you woke, so an Isha you prayed before sleeping is valid. If you were on your period and wake to find yourself clean, it counts as ending when you went to sleep.
Haid is established when blood reaches the outer farj. For a woman who sleeps clean and wakes bleeding, haid is counted from waking; for a menstruating woman who wakes clean, purity is counted from when she slept. Both are taken as a precaution for the prayer.
The four schools agree on this.
Explanation
Many periods start or stop at night, and the timing changes what you owe. The Hanafi rules take the cautious side in both directions, so that a prayer is never lost through doubt.
When it starts:
- A period begins when blood reaches the outer part of the private area, the part washed in istinja. Blood felt inside that has not come out does not start it.
- Woke up to find blood: count it from the moment you woke. A prayer whose time ended before you woke is owed if you had not prayed it; a prayer in whose time you woke is lifted.
- Noticed blood during the day: count from when you saw it, unless you know it started earlier.
When it ends:
- A period ends when you see pure white discharge or complete dryness. See when to do ghusl after your period.
- Went to sleep bleeding and woke up clean: count the purity from when you went to sleep. So if you slept in Isha time, Isha is owed; pray it after your ghusl.
- Checked before sleeping and saw white: your period had already ended, so the prayer of that time is owed.
Both rules come from the same idea: when unsure, take the view that keeps your prayers safe. Keep a note of start and end times; it also makes counting the 3 and 10 day limits easy. See minimum and maximum days of a period and making up prayers around your period.
The evidence
Quran, Surah Al-Baqara 2:222
وَيَسْـَٔلُونَكَ عَنِ ٱلْمَحِيضِ ۖ قُلْ هُوَ أَذًۭى فَٱعْتَزِلُوا۟ ٱلنِّسَآءَ فِى ٱلْمَحِيضِ ۖ وَلَا تَقْرَبُوهُنَّ حَتَّىٰ يَطْهُرْنَ ۖ فَإِذَا تَطَهَّرْنَ فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ ٱللَّهُ ۚ إِنَّ ٱللَّهَ يُحِبُّ ٱلتَّوَّٰبِينَ وَيُحِبُّ ٱلْمُتَطَهِّرِينَ ﴿٢٢٢﴾
And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you. Indeed, Allah loves those who are constantly repentant and loves those who purify themselves."
Menstruation has its own rulings.
Quran, Surah Al-Baqara 2:286
لَا يُكَلِّفُ ٱللَّهُ نَفْسًا إِلَّا وُسْعَهَا ۚ لَهَا مَا كَسَبَتْ وَعَلَيْهَا مَا ٱكْتَسَبَتْ ۗ رَبَّنَا لَا تُؤَاخِذْنَآ إِن نَّسِينَآ أَوْ أَخْطَأْنَا ۚ رَبَّنَا وَلَا تَحْمِلْ عَلَيْنَآ إِصْرًۭا كَمَا حَمَلْتَهُۥ عَلَى ٱلَّذِينَ مِن قَبْلِنَا ۚ رَبَّنَا وَلَا تُحَمِّلْنَا مَا لَا طَاقَةَ لَنَا بِهِۦ ۖ وَٱعْفُ عَنَّا وَٱغْفِرْ لَنَا وَٱرْحَمْنَآ ۚ أَنتَ مَوْلَىٰنَا فَٱنصُرْنَا عَلَى ٱلْقَوْمِ ٱلْكَٰفِرِينَ ﴿٢٨٦﴾
Allah does not charge a soul except [with that within] its capacity. It will have [the consequence of] what [good] it has gained, and it will bear [the consequence of] what [evil] it has earned. "Our Lord, do not impose blame upon us if we have forgotten or erred. Our Lord, and lay not upon us a burden like that which You laid upon those before us. Our Lord, and burden us not with that which we have no ability to bear. And pardon us; and forgive us; and have mercy upon us. You are our protector, so give us victory over the disbelieving people."
Allah does not hold us to what we could not know.
Muwatta Malik 128
Graded in the Muwatta... لا تعجلن حتى ترين القصة البيضاء ...
"Aishah told the women: do not be hasty until you see the pure white discharge."
The end of the period is known by a clear sign, checked with care.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, chapter on haid. If she wakes to see blood she is menstruating from waking; if she wakes pure, she is pure from the time she slept, as a precaution.
- Nur al-Idah with Maraqi al-Falah, Kitab al-Taharah, chapter on haid, nifas and istihadah. Haid is established when the blood reaches the outer farj.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.