Well spiders do have 1 hp, they are in the monster manual. So if you've established that there are spiders in the area, you would indeed have them stymie the sleep spell? And I can carry a box of spiders around to protect myself?
Some spiders are big enough that they
do have 1 or more hit points. Such creatures are 'tiny' in game terms, about the size of a cat. Carry a box of them just like you would (or wouldn't) carry a bag of rats or a cage of chickens.
There are many much smaller spiders that don't have game stats, because they are 'creatures' but are not 'monsters' as described in the MM, the section I quoted not long ago. But if a 'spider' IS one from the MM, or a rat or chicken, and you got attacked by some....why
wouldn't sleep work? According to the MM, they are 'monsters'; creatures that you can interact with and potentially fight and kill, so why would offensive magic not work on them?
Not
all creatures are 'monsters'. They don't all have game stats. But the ones in the MM
are both 'creatures' and 'monsters' by the game's own definition, and interact with spells and other game elements as written.
So, the 20-foot radius circle of your
sleep spell in the woods is likely to have hundreds of creepy-crawlies, including various species of arachnid, that are so small that they (while still being 'creatures') are not 'monsters' and have no game stats, including hit points. Any that
are big enough to be a 'monster' interact with the game normally, including having their hit points subtract from the
sleep pool. But there are unlikely to be hundreds of cat-sized spiders you didn't notice, and it'd be a jerk DM that made some spontaneously appear every time you cast
sleep, or who defined the ones that
are big enough to have stats and be defined as 'monsters' count as 'not creatures' while they are biting hit point-sized chunks out of you.