Date: 202111
Local time of day: 1801-2400
Location: ZAU.ARTCC, IL
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZAU
ATC / advisory: Center ZAU
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Fatigue; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I am a hard-core AM Pilot.  However; I picked up a two-day PM out of ZZZ the day after I finished a three-day AM out of my domicile in ZZZ1. Since it was out of ZZZ; I did not have to commute; however; I did wake up at approximately XA:30 am that day. We were at FL290 and cleared to cross POGGY at 240.  As PF; I set the altitude in the FMC and verified VNAV PATH.  However; I failed to set 240 in the MCP altitude window.  During that call from Chicago Center; the FO was in the middle of updating weather; and performance data.  We also got an ATC frequency change shortly after.  Approximately five miles from POGGY; while at FL290; we were directed to descend to FL240.  At that point we realized the mistake.  ATC never mentioned anything about the lack of descent; but they gave us headings and altitudes to get us back on the STAR.  The remainder of the flight was uneventful.


Second narrative: We were cruising at FL310; shortly before planned top of descent on the ENDEE 4 Arrival.  Captain was PF; and I was PM.  As it was the top of the hour; I was 'heads down' and busy setting up for the arrival into MDW; requesting and writing down the most current ATIS; gate information; and obtaining landing data.In the middle of the process; we received a clearance from Chicago Center to cross POOGY at FL240; and maintain 300 KIAS in the descent.  I read back the clearance to Center; and watched the Captain program the information into the FMC.  He verbalized the information; I verified it; and he pressed the execute button.  I looked at my FMA and verified the proper flight modes (LNAV and VNAV PATH); and resumed setting up for the arrival.After a couple of minutes; as I was finishing up the arrival preparation and ready for the Captain's approach briefing; we received a call from ATC (next sector Controller) with a clearance to descend to FL240.  I thought that we had already started the descent; but when I glanced at my PFD and MFD I quickly realized that we were still at FL310; and only about five miles from POOGY!  The Captain and I both looked at the altitude selector on the MCP; which was still set at 31;000 feet.  He immediately set it for 24;000 feet; and we began a rapid descent at 300 KIAS in LVL CHG; with speedbrakes deployed.  We crossed POOGY about 5;000 feet high; and were then cleared down to FL190; with a speed reduction to 250 KIAS.  We were still well above the planned descent profile (a strong tailwind was not helping) and I advised ATC that we would be high at the next crossing waypoint.  Shortly thereafter; we rejoined the planned descent profile; and landed without further incident.As PM; I should have caught the error of failing to set the altitude selector for the descent.  After I had observed both altitude and speed correctly entered into the FMC and executed; my mind went back to the tasks I had been working on when interrupted by the ATC call.  As it had been a long day for me (I had been awakened earlier than normal for a service call at the house in the morning); I was a bit tired already; and lost a bit of focus.  I was also distracted by the other tasks I was busy with in preparing for our arrival.  As a result; I forgot to verify the crucial step that the altitude selector had been properly set.


Synopsis: Air carrier flight crew reported an altitude deviation during descent to MDW while on the ENDEE 4 Arrival.  The Captain reported they entered the correct information in the FMC; but failed to set the altitude in the MCP altitude window.  Both pilots stated fatigue was a contributing factor.