Fatigue was cited as a factor when an A319 missed a crossing restriction.

Narrative

I was the pilot not flying (PNF). We were cruising east-bound at FL370 about 40 miles west of SLT. ATC assigned us to cross SLT VOR at FL350. I acknowledged on the radio and set the new altitude in the FCU window. The PF programmed the crossing restriction in the FMS. I noted the white top of descent arrow appear on the route many miles ahead. At this point it had been a very long day and I was feeling tired and fatigued. I allowed myself to get distracted and didn't notice that we had flown past the white arrow. Over SLT; still at FL370; I immediately realized we had missed the descent point. The PF immediately descended the aircraft to FL350 and I reported the altitude change (leaving FL370 for FL350) to ATC. We arrived at FL350 a few miles past SLT. ATC acknowledged our descent report. There did not appear to be any conflict with other aircraft due to our late descent. With this error I am reminded that; no matter how tired I am; I cannot allow myself to get distracted at such a critical time. Knowing that there is an assigned descent point ahead; I need to remain vigilant to execute the descent as assigned. I also think I allowed myself to relax because I was the PNF.