Ashta means Eight and Prahar means a period of 3 hours. As the day is divided in 24 hours it so do the prahar of each 3 hours and hence there are 8 prahars. And the duration from morning 6pm to the next day morning 6am is called Ashta prahar.
Praharas by time and ragas for each of them:
Prahar 1 - 6am to 9am:
Vairav, Bengal Vairav, Ramkali, Bibhas, Jogai, Tori, Jaidev, Morning Keertan, Prabhat Bhairav, Gunkali and Kalingara,
Prahar 2 - 9am to 12pm:
Deva Gandhar, Bhairavi, Mishra Bhairavi, Asavari, JonPuri, Durga, Gandhari, Misra Bilawal, Bilawal, Brindawani Sarang, Samant Sarang, Kurubh, Devanagiri
Prahar 3 - 12pm to 3pm:
Gor Sarang, Bhimpalasi, Piloo, Multani, Dhani, Triveni, Palasi, Hansknkini
Prahar 4 - 3pm to 6pm:
Traditional Keertan of Bengal, Dhanasari, Manohar, Ragasri, Puravi, Malsri, Malvi, Sritank and Hans Narayani
Prahar 5 - 6pm to 9pm:
Yaman, Yaman Kalyan, Hem Kalyan, Purvi Kalyan, Bhupali, Puria, Kedar, Jaldhar Kedar, Marwa, Chhaya, Khamaj, Narayani, Durga, Tilak Kamod, Hindol, Misra Khamaj Nata, Hamir
Prahar 6 - 9pm to 12am:
Deshkar (Night), Desh, Desh Misra, Sorat, Bihag, Darsh, Champak, Misra Gara, Tilang, Jai Jawanti, Bahar, Kafi, Arana, Megah, Bagishari, Rageshwari, Malhal, Miya, Malhar
Prahar 7 - 12am to 3am:
Malgunji, Darbari Kanra, Basant Bahar, Deepak, Basant, Gauri, Chitara Gauri, Shivaranjini, Jaitsri, Dhawalsri, Paraj, Mali Gaura, Maad, Sohani, Hans Rath, Hans Dhwani
Prahar 8 - 3am to 6am:
Chandrakos, Malkos, Gopika Basant, Pancham, Megh Ranjini, Bhankar, Lalita Gauri, Lalita, Khat, Gurjari Tori, Barati Tori, Bhaopal Tori, Prabhati Kirtan