It could be helpful to use it on Wizard Monkeys before they ramp up too much damage, but it may be better to save a Bed Time in case the player brings out another control card such as Jungle's Bounty Druid, or get more aggressive with swarms of bloons to overwhelm them.
Gwendolin's Heat It Up and/or playing on Game Night can protect Monkeys with exactly 30 Attack Power against a Bed Time. For instance, Blade Maelstrom.
Bed Time is an Uncommon and is the most common Monkey remover that is a Power, so will be more likely to be drawn than most other Monkey removers.
Some players may use Weapon Enhancer Bloon to limit the capability for opponents to use Bed Time on low ATK Monkeys, mainly Jungle's Bounty Druid.
Competitive players tend to not use Bed Time, as other Monkey removals being more efficient later in the game, and most control decks don't rely on low ATK or non-attacking Monkeys.