There are 3 time periods during the day during which it is not permissible to pray any extra prayers (sunnet namaz, qazha/make-up namaz, nafileh/extra namaz).

Also, it is highly recommended to avoid sleeping during the 1st and 2nd time periods (45 minutes after sunrise, 45 minutes before sunset).

Istiwa. The 45-50 minute time period after the sun rises. (After the time for Fejr/Sabah/Morning namaz is finished)
The 45 minute time period before the sun sets. (The last 45 minutes of Asr/Ikindi/Afternoon namaz. Before Magrib/Aksam/Evening namaz begins)
The 30-40 minute time period when the sun reaches its peak. (Before Zuhr/Öğlen/Noon namaz begins)

The kerahat time applies only to the extra prayers, not the fard! If you have left your fard (required) namaz to the kerahat time, still perform it!