Today the topic of Brick Game came up again. For those that don't know, it's basically a cheap knock-off of the Game Boy with a 10×20 screen (as well as some dedicated HUD) and several games in it (which ones depends on the model). Just look it up. One interesting thing about the Brick Game is that it included sleep mode: if you turned off the system while the game was paused, when turning on it'd be back in the same exact state as when it was paused instead of resetting. Every game. Not bad for what's supposed to be a cheap knock-off.
But that's the issue. While I don't know the exact release date of the first model it's safe to say that at least in the mid-'90s it was already out there (and possibly earlier), but I can't think of any other handheld predating it that had sleep mode as well. The Game Boy line acquired the ability with the GBA, which was 2001. As far as I know Lynx didn't have it, and Game Gear definitely didn't either. The WonderSwan had Dicing Knight, but that one did it since it already had to save the state of the entire dungeon in the first place (so it was more like hybernation than sleeping). No idea about the Neo Geo Pocket, but it doesn't matter since Brick Game still predates all of them.
So huh, does anybody here know if there's any earlier handheld that had sleep mode that I may be missing? Because the idea that a cheap knock-off was the first handheld ever to include sleep mode seems ridiculous, but what I know so far seems to hint at that.