Time machine backup irregularities on High Sierra

Time Machine frequently will not start backups on High Sierra on my Mac Book Pro. It worked fine previous to the High Sierra installation. Under High Sierra the sequence is: Start backup; the Mac sleeps (can't seem to prevent this despite numerous attempts). After logging back in, when I attempt to restart the backup, Time Machine just punts. If I restart the Mac Book, it appears that I can run Time Machine until the next sleep which it seems that I cannot lengthen.