B737 Undifferentiated or Other Model
Flight Crew Last 90 Days 251; Flight Crew Type 5000
Fatigue; Situational Awareness
Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors
We leveled at FL380 on our way to LAS after being assigned FL390. We had leveled at FL380 and been level for approximately 10 minutes.We discovered this after a frequency change. I was the Pilot Flying. However; when we changed Center Control frequencies; I took the call. Previously; during this flight; the Captain had been working the radios. When I checked-in on the new frequency; I stated our altitude (FL380) and then; as is my habit pattern; looked at the FMC Altitude window and noticed that it was different from our current altitude. The FMC altitude window said FL390. So; I checked in by saying 'Seattle; (call sign) FL380 climbing FL390.'  At that point; I had not noticed that we had been level at FL380 for approximately 10 minutes already. I had simply assumed we were still slowly climbing to our final cruise altitude.Seattle called back and asked us to confirm our altitude. At that point; I noticed that we weren't climbing - that we were level. I then realized something must be wrong. I looked at the FMC Altitude window again. It said 39;000 feet.  I told Seattle it looked like we were climbing to FL390 but we were level at FL380. Seattle said to climb to FL390. I affirmed that clearance. The Captain then jumped on the radios and confirmed the clearance.We climbed to FL390 without incident. We never received any kind of TCAS information. We were never informed by ATC that our altitude had resulted in a traffic conflict.In retrospect; I realize that I had probably entered FL380 into the FMC on the ground when we got into the gate and before we got our new paperwork. We were late and I was trying to get ahead. I entered an incorrect direction of flight altitude and then forgot that I had done so later.This was the last leg of an EXTREMELY taxing four-day trip that would have been ILLEGAL prior to FAR 117.  I was more tired than I realized. I am writing this an hour after the trip blocked in and I am exhausted.  I cannot over-emphasize the importance of fatigue in situations like this. I didn't realize how tired I was.
A B737 First Officer reported unknowingly leveling 1;000 feet below cleared cruise altitude for 10 minutes; and stated fatigue was a factor.