Narrated Abu Huraira:
The Prophet (sallallaahu alayhi wa sallam) said, "Religion is very easy and whoever overburdens himself in his religion will not be able to continue in that way. So you should not be extremists, but try to be near to perfection and receive the good tidings that you will be rewarded; and gain strength by worshipping in the mornings, the afternoons, and during the last hours of the nights."
[Sahih al-Bukhaari Hadith No. 39]
Being extreme means indulging in optional good deeds to such an extent that one misses deeds which are better.
An example is a person who prays all night. Then, he falls asleep and misses the Fajr prayer in congregation. His voluntary worship prevented him from what was better, namely praying the Fajr prayer in congregation.
Another example is a person who memorizes the Qur'aan all night. Then, he falls asleep and misses the Fajr prayer. This is haraam.
Finally, one should not inquire about matters of the unseen regarding which Islamic legislation requires one has Imaan and does not inquire about their modality.