This guide lists the solutions to all levels and provides information on how to obtain all the achievements.
ATTENTION! It’s spoiler-heavy! It’s more fun to play the game without referring to these screenshots.

Find 50 differences with less than 10 wrong clicks
Currently missable! (the developer promised a fix with the next patch).
You need to have less than 10 misclicks during the first 50 differences that you click, so in effect the first 10 levels / first world. Resetting this requires deleting your game progress:

Complete the first level with 5 stars
You should get this with ease. If you struggle finding any object, refer to the solution below.

Find all the differences from the game
Complete all the game’s levels. If you really can’t find an object on any level, solutions are below.

Rotate any level 20 times
You should get this while playing just from tapping A & D to rotate.

Find 5 differences without using zoom on any level
Self-explanatory and can be done on the first level. Only rotate with A & D, don’t press W or S.

Find 5 differences in less than 15 seconds on any level
Can easily be done on the first level. Just refer to the solution below. Can be combined with the previous.

Find 5 differences in less than 60 seconds on any level
Can easily be done on the first level. Going for the previous achievement provides this one as well.

Find all differences on any level without wrong clicks
Can easily be done on the first level, just make sure not to misclick an object. Can be combined with the previous ones.

Stay in a level or more than 15 min
Self-explanatory, just go AFK. The game likely has to be in foreground / focus.

Play any level without any sound
Disable sound effects and music in the bottom right of the main menu, then launch any level. Since it unlocks on level start, you can exit out again.

Click wrong more than 100 times

Fail to find differences more than 500 times
These two unlock after you accumulate a lifetime total of 100 / 500 misclicks on objects. Note that environment misclicks, like clicking the water, do not count. Usually there will be a sound effect played for a misclick. You can use an auto-clicker tool to speed this up and avoid RSI.
You can go AFK for 15 minutes while the auto-clicker misclicks to combine it with Are you asleep?.
Bucket
Lotus flower
Tree log
Rock
Bird wings
Package
Boat
Bucket
Fish
Lotus leaf
Bird
Sword
Tree log
Fish
Spear
Crate
Stone
Blue bottle (inside the cart)
Flag
Shield (on the back of the soldier)
Spider
Red leaf
Tree log
Blue butterfly
Egg (on the tree)
Shield
Bucket (behind the horse)
Water jar (next to the well)
Red flag (on the cabin)
Water jar (next to the cabin)
Lotus leaf
Lotus flower
Lotus leaf (left side)
Bucket
Lotus leaf (right side)
Red bag
Steak
Umbrella
First aid kit
Bicycle
Pillar element (bottom)
Pillar element (left)
Native Indian
Pillar element (center)
Pillar element (top) – wing
Lotus flower
Butterfly
Mushroom (purple)
Mushroom (grey)
Lotus leaf