Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Confusion; Fatigue; Physiological - Other; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Human Factors
While waiting for the airplane to be towed to the departure gate; I ate a heavy breakfast and did not consume any coffee as this was an Air Carrier X operation and I anticipated coffee being readily available on the plane.  In spite of my request; the FA never made any coffee.  ATC issued a 15 degree heading change for traffic.  I made the heading change without incident.  Subsequently; I began to feel a bit drowsy.  After approximately 5 minutes; ATC issued 'Direct SUNED' fix.  I selected direct SUNED in the FMS and activated after PM confirmed the change.  I forgot to select the NAV mode on guidance panel and we continued on the previous heading for 3 more minutes.  I snapped to attention ATC reissued the Direct SUNED clearance and after a moment of confusion as to why the clearance was reissued; I realized that the guidance panel was still set to HDG mode.  The proper NAV mode was engaged and we proceeded direct to SUNED without incident or any further mention of the issue by ATC. Driven by adrenaline; I remained mentally aroused and vigilant for the remainder of the flight.Cause - PF's Postprandial lassitude caused by failure to ingest a suitable amount of caffeine following heavy breakfast.  PM inattention/distraction was also a factor.Ensure personal availability of postprandial drowsiness countermeasures (aka Coffee) and remember to verify changes on flight mode annunciator.
Air carrier First Officer reported a track heading deviation during departure from SLC airport.  First Officer cited feeling drowsy due to not consuming caffeine after a big breakfast may have contributed to the event.