I consider that ebb and flow a totally acceptable part of human reality - as long as you find yourself not self-destructively procrastinating in the lower energy times, it seems sustainable to go at 75% (making up a number) sometimes and 125% later when the time or energy is available to you! I like feeling like it's my choice to go at 125% though - personal agency has always been key to keeping myself invested too.

The behavior modeling you describe in your post most resonated with me at the present moment - as I become more of a senior employee, I feel pressure to perform from management, but also pressure to model sustainable behaviors. The Netflix culture book covered some of this with respect to time off. Top-down is important for culture setting.

I am holding up well, thank you! But ~6 months at the new company, I wasn't holding up great and needed to seriously rethink my time management (hence a lot of this being fresh in my brain). I'm fortunate to have been able to fix this to the best of my ability before my son arrived - but at a company with a pride for startupiness (read: at times a total lack of planning), the balance I have needs pretty regular maintenance.