Check OS layer first, check the load average, IO and everything.
Also look at the db site.. Most of the time, we see the users waiting for the db actually.
For db performance analysis, start with AWR.
Create an AWR report which is built the AWR data produced during the problematic interval.
After creating the AWR report, analyze it and try to find the root cause there.
For understanding the AWR analysis, you can start by reading ->
http://ermanarslan.blogspot.com.tr/2014/05/awr-analysis-part-1.html