Do i have to rest...

[Sharkon]

...to gain access to new spell slots when i have just leveled up ?

 

[Starbuck_II]

...to gain access to new spell slots when i have just leveled up ?

If you are an arcane caster, yes.
Divine just need to find appropriate day of time.

 

[StreamOfTheSky]

In my group, you can't level up at all until you rest. It certainly helps the DM not have to keep constant track of the xp earned when someone's close. Not to mention if they level up mid-day, the xp they get from the same CR encounter will then change, making it even more confusing.