Stay awake
One difference with my Mac that I’ve noticed is that after a little inactivity the screen dims and then shortly afterwards the screen saver kicks on. Now I know this can be modified in system preferences but there are a number of conditions to adjust and it isn’t always convenient to change them depending on your usage. In particular, if you are watching a video the Mac doesn’t notice that the machine is being used and will dim the screen even though you are watching it.
A simple program called Caffeine will keep your computer awake. It sits in the menu bar and if you click it your Mac won’t go into the screensaver (the coffee cup is filled so you can see your machine is now caffeinated). Click it again and you will get the original behaviour. Pretty simple but useful. The program is free.