Date: 202010
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Commercial Fixed Wing
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft TA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Environment - Non Weather Related
Primary problem: Human Factors

Narrative: We were descending on the ZZZZZ4 arrival north of ZZZZZ when ATC issued a traffic advisory of an Aircraft Y at 12 o'clock. The FO was pilot flying and I was looking for the traffic along with the Relief Pilot. I reported the traffic in sight and meanwhile the pilot flying descended below 9;000 before reaching ZZZZZ. The Controller said maintain 9;000 and maintain visual separation with the Aircraft Y and we returned to 9;000.If the Controller had said maintain 9;000 and then issued the traffic; I believe the error would have been prevented. Fatigue was certainly a factor.


Second narrative: We were descending on the ZZZZZ4 arrival. Approaching the ZZZZZ fix we were asked to slow to 210 on the speed early. ZZZZZ shows a 210 speed at 9;000 feet. We were asked to slow at approximately 10;500 feet. As pilot flying I dialed in 210 on the speed pressed flight level change and went heads down to the FMS to insert 210 on the speed. My plan was to repress the VNAV button after inserting the 210 speed. But; as I'm heads down TCAS announces Traffic; Traffic. All three of us go heads up and out looking for the traffic. We spotted the climbing traffic as it passed in front of us. About this time ATC ask if we had traffic and to climb to 9;000. I looked and we had descended through 8;500. We acknowledged we had the traffic in sight and immediately climbed back to 9;000 feet. We realized that I never reengaged VNAV after inserting the new 210 speed in the FMS computer thus the autopilot continued to descend toward the 6;000 foot altitude on the arrival. The rest of the arrival and approach were normal.Our wakeup call was at XA:30 in the morning body clock time. We had been flying for approximately X hours 45 minutes with each of us having a sleep break.This was my second pilot flying trip in over 7 months. This was the Captain's first flying trip in 7 months.Going to flight level change while still on a descend via arrival was a mistake on my part. I feel lack of flying and fatigue both contributed to this.The ATC early slow call very close to a fix interrupted our flow on the arrival. The TCAS Traffic call out caused all of us to look away from the inside auto flight situation.This was my second pilot flying arrival into ZZZ. It was the Captain's first arrival.Lack of flying because of the COVID virus really hurts our overall proficiency. Going out to the simulator more often for situational training not less would help some of this. I have heard we are going to once a year training instead of every nine months. We have also moved landing currency to 120 days from every 90 days. Both of these new policies hurt our proficiency. Otherwise this was just a situation where several small factors added up to an off altitude incident.


Synopsis: Air carrier flight crew reported an altitude deviation during arrival resulting in an airborne conflict.