Ahsan Allahu ilaikum---If a person sleeps through fajr (after giving effort) and wakes up. But after waking up- is it a major sin or kufr (generally) to go back to sleep due to laziness/tiredness instead to instantly pray? Because the brother thinks theres no sin- so he sleeps again and prays later.

Ustadh AbdulAziz al-Haqqan

Seek a doctor for waswas.