# Linear mixed models with correlated fixed effect

I am experiencing linear mixed models and I have a fundamental questions:

Is it allowed to have correlated fixed effects? I.e. I measure the activity performed during a day in step and I want to relate it to the quality of sleep measured as total time slept TTS, number of sleeping epochs NSE and duration of sleeping epochs DSE. These variables are ordinal variables representing 4 interval ranges. Each subject is assessed several days.

My model would be

Step ~ TTS + NSE + DSE + (1|Subject)


but I know that NSE and DSE are correlated: i.e. the higher number of sleeping epochs the shorter these sleeping epochs will be.

Can Effect2 (NSE) and Effect3 (DSE) be correlated? (0.8) What is the proper way to deal with this situation? What if I have more than 2 correlated effects?

Should I fit 2 separate models (one for each of the correlated effects)? Should I include more random effects?