ZLA Center Controller reported they were working busy combined sectors even though there was sufficient staffing to split the sectors when they failed to notice a VFR aircraft under their control had deviated in the R2502E.

Narrative

I was working Sectors 06/16 combined at the end of the night. I had a VFR Aircraft X from LAS Approach Control. He flew into restricted area R2502E. I didn't see it in time. I believe I didn't see it in time to keep him out of that restricted airspace because the sectors were combined up when they should not have been. I felt very busy. By the time I saw him in the restricted airspace; I put him on a 160 heading to turn him out. I didn't really feel like I had time to explain things to him so I just kept him on the heading until he was clear; and then told him to proceed back on course once clear. I'm writing this report because I'm tired of coming to work and working sectors combined when it's too busy. The CICs (Controllers in Charge) and Supervisors are under pressure from Controllers to combine up sectors; and then they won't split it back off for the last few busy pushes of the evening.I especially notice it on my overtimes. To me it's especially dangerous those shifts because the [consecutive] night swing shifts are full of 'young' controllers with very little amount of experience as CPCs (Certified Professional Controllers). They combine up too soon and handle too much on their own with no D sides; all just to roll breaks. We had the staffing and 06/16 should have been de-combined. The volume is too high and you can't even see data blocks over Las Vegas. There's a 'community' pressure to keep sectors combined and just handle it; so people can have more breaks. Another thing that added to the complexity of the session was that I can't hear my aircraft because everyone in the area is laughing and yelling so loud. The volume of the area gets really out of hand and I've had pilots tell me multiple times on frequency that it sounds 'like a party' in there. Or they make comments about the noisy background. Supervisors and Management are scared and intimidated to enforce professionalism. It's been going on for years and the most we have done is 'professionalism' briefings given to the workforce as a whole. Nothing changes from prostandards presentations. I would recommend better training on the importance of keeping sectors split; even if you are feeling pressure or comments from the CPC's to combine up. I would also recommend an attempt to control the volume in the operation so we can't hear people laughing and screaming in the background of our control facility while I'm keyed up trying to control traffic.