Flight Crew Air Transport Pilot (ATP)
Fatigue; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors
While heading into BTV we were on vectors to RNAV GPS Z Rwy 33. We were at 5500 feet and on a heading to intercept the final course inbound. While on this heading we were cleared for the approach. We intercepted the inbound course and proceeded with the approach. What I failed to realize is that we were outside NIDUQ. The minimum crossing at NIDUQ is 5400 feet. We were lower than the required 5400 ft and just about to level at HONIB when approach stated that we descended below the required crossing restriction of 5400 feet at NIDUQ. At this point; we had the airport in sight and told approach that we had the airport in sight. He cleared us for the visual and we continued and landed safely without issue.This was the third night. I was tired as I didn't sleep well the night before and I didn't get enough sleep during the day. I definitely tried to sleep during the day; but only slept about 3 hrs. Once on the ground and parked at the gate. I dug into how I missed the fix. I reloaded the approach with vectors and realized that it doesn't load any fixes past HONIB. I have learned my lesson and will always select an IAF instead of vectors. I was also complacent in thinking that it was going to be a visual approach with the RNAV as a backup.
CRJ-200 First Officer reported descending below charted altitude on approach to BTV citing fatigue as a contributing factor.