My advice - be very careful to distinguish between being 'very tired' and 'fatigued'. I suggest you look up a few definitions for fatigue via Google?
It is accepted by the CAA that it is normal and to be expected to be pretty tired after a max duty/series of, but any company wil look closely at a claim of fatigue and your social life, domestic arrangements, home to duty distance/time etc may well come under scrutiny.
As has been said above IF YOU ARE GENUINELY FATIGUED, DO NOT FLY. That is what the ANO (UK) says. It is the law and it is your responsibility. The company may well ask for a doctor's report, as well, to confirm 'fatigue'. Insomnia may well be considered as a sickness.