Fatigue and tiredness is a very personal thing.
For instance I can survive with about 3-4 hours sleep a night whilst carrying out 4 or 6 sector duties for 5 days. For somebody that needs more sleep than me, that amount of sleep could lead to fatigue.
About a year ago I had a week when I hardly slept (nothing to do with my roster), probably about 30 minutes a night, and by day 3 I was completely f****d. I called in sick for the last 2 days AND filed an ASR. I had no backlash from the company.
It is NOT up to the company to decide if you are fatigued or not. And it doesn't matter what kind of roster you have.
If you feel fatigued then you are. Call in sick and if you feel it's necessary file an ASR.