Wow. Once I start doing this kind of work I both have a hard time stopping (as I find working on such problems addictive, both in good ways and in bad) and feel like it takes at least a couple hours of loading state into my brain before I could even begin to be working "at speed" on a hard problem, which means I actively try to schedule my life such that I either can spend a solid zero hours in a day working on hard mental tasks ;P or make myself ready to work until dawn (at which point I simply feel bad I am still awake sometimes more than truly feel like I am ready to stop).