To the OP: do you sleep well? By that I mean do you get ~ 7 hours uninterrupted sleep every night? Go to bed about the same time every night, get up about the same time each morning? Feel rested once you're up?
How tall are you?
@ 67 & 230 lbs if you're not 6'-2" you're carrying ~ 40 lbs too much. That's gonna tire you out even if you're sleeping well.
My take is you should talk to your PCP about what you're experiencing, get their take on maybe getting a 'sleep test'.
We need Oxygen 24/7, even when sleeping. Sleep apnea – a failure of the body to keep up with proper respiration as we age - will affect the entire body but most importantly the cerebral / cardio systems that keep us alive and functioning. Doing 'nose breathing' alone isn't enough to circumvent the potential lack of Oxygen reaching your brain when you're asleep if there's something affecting the quality of sleep you're getting every night.