Usually battery consumption issue or related issues causing phone over-heating during sleep tracking are not caused by the sleep tracking directly.
Battery saving mode currently resumes full tracking before the smart wake up period in order to find the best moment for your wake up, so the tracking uses up just a fraction of the battery consumption for the whole night. If the battery would drop under your defined stand-by threshold (default: 10%) the battery saving mode will re-occur.
In most cases sleep tracking itself is not consuming much battery (usually around 1-2% per hour of tracking). But because we hold a wake lock (keeping the phone awake) any other usually badly written apps may access the CPU extensively during the sleep tracking time and the consumption is anyway mistakenly accounted to the app holding the wake lock (Sleep as Android in this case). We would suggest checking which services are running before you get to sleep and try to kill some of them or reboot your phone prior tracking. For more details, please read Troubleshooting.