Hi
It is OK. As described in the vSphere Documentation here: vSphere 5.5 Documentation Center - vSphere HA Provides Rapid Recovery from Outages
It protects against a server failure by restarting the virtual machines on other hosts within the cluster.
Since it is restarting the VMs, it means the VM will be stopped/powered off and restarted/powered on in the available ESXi hosts - downtime is required.
If you need an availability features in vSphere that has higher level of availability, then you can explore Fault Tolerance (FT) vSphere 5.5 Documentation Center - vSphere Fault Tolerance Provides Continuous Availability
"vSphere HA provides a base level of protection for your virtual machines by restarting virtual machines in the event of a host failure. vSphere Fault Tolerance provides a higher level of availability, allowing users to protect any virtual machine from a host failure with no loss of data, transactions, or connections."
Review the Fault Tolerance Checklist on its requirements and limitations: vSphere 5.5 Documentation Center - Fault Tolerance Checklist