Not enough sleep or disruptions to the sleep-wake cycle (such as those that may occur with shift work or travelling to a different time zone) cause the physiological state known as fatigue. Staying awake for 24 hours leads to a reduced hand-to-eye coordination that is similar to having a blood alcohol content of 0.1.
There are 13 hearts in a standard 52 card deck. There are also 13 diamonds, 13 spades and 13 clubs.
When your Home City gains a level, you can purchase one more card and one more customization thing. No matter what level you're up to, you can only take 20 cards into battle with you. The rest you own are inactive, but you still own them. The 20 active cards are collectively called the deck, and you can have three decks saved per Home City. Decks can be altered anytime between matches however you wish, and you're not required to have three or anything.