Can this be considered wide sense stationary? I was discussing this problem with one of my classmates.
The picture shows a recording of the heart rate during before and after sleep.


Can the whole process be considered wide sense stationary? (I say yes because the mean is approximately constant, but how can I estimate the autocorrelation?)
Can the whole process be considered ergodic in autocorrelation and mean? (I'm not sure at all about this)
for 1, a random process is WSS if its autocovariance and mean ensemble average don't vary as a function of specific time instance, just lag.  As mentioned before, it's hard to conclude this from simply one realization of the process about the ensemble average.

However for 2, you can at least estimate what the autocovariance and mean are if the process is ergodic (which, unfortunately, you can't conclude with just a single realization).  If the process is indeed ergodic, the ensemble average is equal to the time average in the limit as your interval of average approaches infinity, which is kind of nice with regards to computability.