I haven't gone into this in detail because my preferred trading times coincide with the UK market anyway, and the FTSE100 has the best spreads, so these are generalisations -
the SB index values are usually very close to the underlying instrument during that instrument's trading hours
BUT
the SB daily ranges are generally much greater than the instrument's daily ranges
THEREFORE
the additional volatility must be injected by the SB quotes outside the underlying's trading hours
SO
this increases volatility, reduces the reliability of TA and increases risk
AND
spreads tend to be widened out when the underlying is not trading, so your overheads are automatically that much higher even if you have a winning record
AS A RESULT,
I tend to not actively trade during the overnight,don't depend on the SB's overnight charts as TA, and avoid leaving buy or sell orders in place overnight as the volatility spikes will very regularly hit them with no real TA reason.
At the moment I am avoiding holding overnight where possible becasue the market is reversing so readily and unpredictably, and if I could get to the position where I enter just 1 trade per day which opens an hour after the underlying marker opens and closes an hour before it closes, I would be happy.