All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Combining two prayers is not obligatory, rather it is permissible when there is a valid Sharee'ah reason for it. What you mentioned about the time of Fajr being close to the time of ‘Ishaa' in itself is not a valid excuse that makes it permissible to combine ‘Ishaa’ and Maghrib prayers. However, if you mean that the time between 'Ishaa and Maghrib is long, then it is not among the reasons that allow combining the two prayers in residence. Thus, it is obligatory upon you to perform the Maghrib prayer on its time and perform the 'Ishaa' prayer on its time, unless the 'Ishaa' is so late that it is difficult for the worshipers to wait for it, as some contemporary scholars have issued fatwas that due to this hardship it is permissible to combine the 'Ishaa' and Maghrib prayers at the time of Maghrib prayer.
If you combine 'Ishaa' and Maghrib prayers at the time of the earlier prayer for a valid reason, then it is not obligatory upon you to sleep after the Maghrib prayer, nor to intend to go to sleep, as you mentioned.
Allah knows best.