void OnEntityTakeDamage(BaseCombatEntity entity, HitInfo info)
        {
                        var Sleeping = BasePlayer.sleepingPlayerList as List<BasePlayer>;
			if (!entity || !(entity is BasePlayer) || !(info.Initiator is BasePlayer) || info.Initiator is NPCMurderer || info.Initiator is BaseNpc || info.Initiator is NPCPlayerApex || entity is NPCMurderer || entity is BaseNpc || entity is NPCPlayerApex)
				return;
                        BasePlayer player = entity as BasePlayer;
                        var attacker = info.Initiator as BasePlayer;
                       if (!player.IsSleeping())
                       return;
                        ulong playerId = player.userID;
                        double timeStamp = GrabCurrentTime();
                        var t = pcdData.pEntity[playerId].LogOutDay + LastSeenSeconds;
                        var f = pcdData.pEntity[playerId].Found;
                        var e = pcdData.pEntity[playerId].Event;
			if (player.IsSleeping() && (pcdData.pEntity.ContainsKey(playerId)) && (timeStamp > t) && (f < 1) && (e))
                         {           
                          PrintToChat(string.Format(lang.GetMessage("found", this), player.displayName, attacker.displayName));
                          pcdData.pEntity[playerId].Found = 1;
                          SaveData();
                          return;                         
}
}

​

For ignoring damage to BasePlayer:

object OnEntityTakeDamage(BaseCombatEntity entity, HitInfo info)
{
    if (entity is BasePlayer)
    {
        return true;
    }

    return null;
}

For ignoring damage from BasePlayer:

object OnEntityTakeDamage(BaseCombatEntity entity, HitInfo info)
{
    if (info != null && info.Initiator is BasePlayer)
    {
        return true;
    }

    return null;
}

info.InitiatorPlayer may also work instead of "info.Initiator is BasePlayer"

BasePlayer​

if (!player.userID.IsSteamId()) return;​

(12:31:50) | 8951308[17051429/8951308] died (Generic)
(12:31:50) | Failed to call hook 'OnEntityDeath' on plugin 'SleeperMark v1.0.1' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.SleeperMark.OnEntityDeath (BaseCombatEntity victimEntity, HitInfo info) [0x00016] in <658f95414c6b4861b1b35b5534c30e39>:0 
  at Oxide.Plugins.SleeperMark.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00467] in <658f95414c6b4861b1b35b5534c30e39>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9affce1cd15c4ec183941adef8db1722>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <4452f821def6406d834e4149849fe7ea>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0
​

        void OnEntityDeath(BaseCombatEntity victimEntity, HitInfo info)
        {
            if (!victimEntity || !(victimEntity is BasePlayer) || !(info.Initiator is BasePlayer) || info.Initiator is NPCMurderer || info.Initiator is BaseNpc || info.Initiator is NPCPlayerApex || victimEntity is NPCMurderer || victimEntity is BaseNpc || victimEntity is NPCPlayerApex)
                return;
            BasePlayer player = victimEntity as BasePlayer;
            if (!player.IsSleeping())
                return;
            ulong playerId = player.userID;
            double timeStamp = GrabCurrentTime();
            var attacker = info.Initiator as BasePlayer;
            if (!player.userID.IsSteamId() || !attacker.userID.IsSteamId()) return;

            var t = pcdData.pEntity[playerId].LogOutDay + LastSeenSeconds;
            var e = pcdData.pEntity[playerId].Event;
            if (player.IsSleeping() && (pcdData.pEntity.ContainsKey(playerId)) && (timeStamp > t) && (e))
            {
            if (useRewards)
            {
                PrintToChat(string.Format(lang.GetMessage("dead", this), player.displayName, attacker.displayName, reward));
                Effect.server.Run("assets/bundled/prefabs/fx/explosions/explosion_01.prefab", attacker.transform.position);
                Effect.server.Run("assets/bundled/prefabs/fx/smoke_signal_full.prefab", player.transform.position);
                pcdData.pEntity[playerId].Event = false;
                SaveData();
                ServerRewards?.Call("AddPoints", attacker.userID, (int)reward);
                if (useAirdrop)
                SpawnAirdrop(attacker.transform.position);
                }
            if (!useRewards)
            {
                PrintToChat(string.Format(lang.GetMessage("NoRewards", this), player.displayName, attacker.displayName));
                Effect.server.Run("assets/bundled/prefabs/fx/explosions/explosion_01.prefab", attacker.transform.position);
                Effect.server.Run("assets/bundled/prefabs/fx/smoke_signal_full.prefab", player.transform.position);
                pcdData.pEntity[playerId].Event = false;
                SaveData();
                if (useAirdrop)
                SpawnAirdrop(attacker.transform.position);

            }
        }
    }​

if (info == null) return;
if (info.Initiator == null) return;
if (victimEntity == null) return;

and only then -> check for NPC and etc stuff​

if (entity is BasePlayer && !entity.IsNpc)
{
    return true;
}

if (info != null && info.Initiator is BasePlayer && !info.Initiator.IsNpc)
{
    return true;
}